CSS Custom Properties  –– all'ennesima potenza

Francesco Improta

I'm a product designer
with a love for mountains, astronomy and code.

@zetareticoli

github
twitter

Head of Design @ Citynews

francescoimprota.com

What are Custom Properties?

Add variables to CSS 🥳

Properties or variables?

CSS a programming language?

CSS preprocessors
did the job

// $VARIABLES ---------------------------------------------------------------//

// COLORS ======================================================================

// Base colors -------------------------------------------------------------- //
$color-black: #151010;
$color-steelblue: #274b7b;
$color-white: #fff;

// Gray --------------------------------------------------------------------- //
$color-gray-10: #fafafa;
$color-gray-20: #f5f5f5;
$color-gray-30: #eee;
$color-gray-40: #e0e0e0;
$color-gray-50: #bdbdbd;
$color-gray-60: #9e9e9e;
$color-gray-70: #757575;
$color-gray-80: #616161;
$color-gray-90: #424242;
$color-gray-100: #212121;

// Slategray -----------------------------------------------------------------//
$color-slategray-10: #eceff1;
$color-slategray-20: #cfd8dc;
$color-slategray-30: #b0bec5;
$color-slategray-40: #90a4ae;
$color-slategray-50: #78909c;
$color-slategray-60: #607d8b;
$color-slategray-70: #546e7a;
$color-slategray-80: #455a64;
$color-slategray-90: #37474f;
$color-slategray-100: #263238;

// Accent Colors -------------------------------------------------------------//
$color-accent-magenta: #e91f63;
$color-accent-mint: #00bfa5;
$color-accent-orange: #ffa000;
$color-accent-gold: #edce8e;
$color-accent-firebrick: #ff3030;

// Social colors -------------------------------------------------------------//
$color-facebook: #3b5998;
$color-twitter: #1da1f2;
$color-youtube: #ef450f;
$color-telegram: #2ca5e0;
$color-whatsapp: #25d366;

// Themes --------------------------------------------------------------------//
$color-theme-black: #3b3b3b;
$color-theme-blue: #3277c7;
$color-theme-cyan: #2a9ddb;
$color-theme-green: #50ae55;
$color-theme-purple: #b75dc7;
$color-theme-pink: #f291b2;
$color-theme-red: #f2463d;
$color-theme-ruby: #d92121;
$color-theme-yellow: #ffbe26;

$color-theme-black  : lighten($color-black, 10%);

// Other colors
$color-forestgreen : #228b22;
$color-blue        : #01579b;

// Brand colors selection ----------------------------------------------------//
$color-black-selection: rgba($color-theme-black, .3);
$color-blue-selection: rgba($color-theme-blue, .3);
$color-cyan-selection: rgba($color-theme-cyan, .3);
$color-green-selection: rbga($color-theme-green, .3);
$color-purple-selection: rgba($color-theme-purple, .3);
$color-pink-selection: rgba($color-theme-pink, .3);
$color-red-selection: rgba($color-theme-red, .3);
$color-ruby-selection: rgba($color-theme-ruby, .3);
$color-yellow-selection: rgba($color-theme-yellow, .3);


// UI ------------------------------------------------------------------------//

$global-background-primary : $color-theme-blue;
$background-overlay : transparentize($color-slategray-100, .15);
$background-overlay2 : transparentize($color-slategray-100, .05);

// Background
$color-ui-bg-base: $color-white;
$color-ui-bg-inverse: $color-black;
$color-ui-bg-light: $color-gray-20;
$color-ui-bg-overlay: transparentize($color-black, .45);
$color-ui-bg-secondary: $color-slategray-100;

// Borders
$color-ui-border: $color-gray-40;
$color-ui-border-link: $color-gray-40;
$color-ui-border-dark: $color-black;

// Body
$color-ui-body: $color-black;
$color-ui-body-inverse: $color-white;
$color-ui-body-secondary: $color-gray-70;
$color-ui-body-light: $color-gray-50;

// Buttons
$color-ui-button-primary: #337ab7;
$color-ui-button-secondary: $color-gray-60;

// Forms
$color-ui-form-label: $color-gray-50;
$color-ui-form-input: $color-gray-50;

// Icons
$color-ui-icon: $color-black;
$color-ui-icon-active: $color-black;
$color-ui-icon-inverse: $color-white;
$color-ui-icon-light: $color-gray-60;

// Links
$color-ui-link: $color-steelblue;

// Sponsor
$color-ui-sponsor: $color-accent-orange;



// TYPOGRAPHY ==================================================================

// Line height
$lineheight-base: 1.25;
$lineheight-extra: 1.5;
$lineheight-body: 1.8125rem; // 29px - use along with 'greatprimer' type size

// Type Sizes
$type-canon: 3rem; // 48px
$type-canon-medium: 2rem; // 32px
$type-canon-small: 1.5rem; // 24px

$type-doublecolumbian: 2rem; // 32px
$type-doublecolumbian-small: 1.5rem; // 24px
$type-doublecolumbian-xsmall: 1.25rem; // 20px

$type-doublepica: 1.5rem;// 24px
$type-doublepica-small: 1rem;// 16px
$type-doublepica-small: .875rem;// 14px

$type-english: 1.15rem; // 18px
$type-english-small: .875rem; // 14px
$type-english-xsmall: .75rem; // 12px

$type-paragon: 1.25rem; // 20px
$type-greatprimer: 1.15rem; // 18px
$type-columbian: 1rem; // 16px
$type-pica: .75rem; // 12px
$type-primer: .625rem; // 10px

// Font size 
// $TODO : REMOVE
$font-size-big    : 2.25rem; // 36px
$font-size-xhigh  : 1.75rem; // 28px
$font-size-xbase  : 1.125rem;// 18px
// $END TODO 

// Font Weight
$font-weight-normal: 400;
$font-weight-medium: 500;
$font-weight-bold: 700;
$font-weight-black: 900;

// Font Family
$font-serif-stack: Georgia, "Times New normal", serif;
$font-sans-stack: Helvetica, Arial, sans-serif;

$font-serif: 'PublicoText', $font-serif-stack;
$font-serif-display: 'Publico', $font-serif-stack;
$font-sans-display: 'NeueHaasGroteskDisp Pro', $font-sans-stack;
$font-sans: 'NeueHaasGroteskText Pro', $font-sans-stack;
$font-sans-narrow: 'NovecentoNarrow', $font-sans-stack;

// MEASURES ====================================================================
$baseline: .5rem; // 8px
$space: 1rem;
$gutterwidth    : $space;
$baselineheight : 1.5rem;

// Sizes -------------------------------------------------------------------- //
$size1x: $baseline; // 8px
$size2x: $size1x * 2; // 16px
$size3x: $size1x * 3; // 24px
$size4x: $size1x * 4; // 32px
$size6x: $size1x * 6; // 48px
$size8x: $size1x * 8; // 64px
$size9x: $size1x * 9; // 72px
$size12x: $size1x * 12; // 96px

$size-fluid: 100%;


// Grid ----------------------------------------------------------------------//
$gutter: $size2x;
$gutter-large: $size3x;

// Use $space to set vertical space between elements
$space: $gutter;
$space-large: $gutter-large;
$space-small: $gutter / 2;
$space-xlarge: $gutter * 3;

// Icons ----------------------------------------------------------------------//
$icon-small: $size2x;
$icon-base: $size3x;
$icon-large: $size4x;


// SHAPES ======================================================================

$border-style: solid;
$border-size-base: 1px;
$border-size-medium: 2px;
$border-size-large: 4px;
$border-size-xlarge: 6px;

$radius: 3px;
$radius-rounded: 50%;

// ANIMATIONS ==================================================================

$default-timing: .3s;
$transition-function: ease;


// COMPONENTS ==================================================================

// Header --------------------------------------------------------------------//
$header-small   : 3.5rem; // 56px
$header-medium  : 4rem; // 64px
$header-large   : 4.5rem; // 72px

// Navbar --------------------------------------------------------------------//
$navbar-size: 3.5rem;
$navbar-spacing: 1rem;

❤️ Sass

Preprocessors Flow

Preprocessors variables only have one value at a time

Preprocessors Limits 👎

Each preprocessor has own syntax

Additional setup

Recompilation after changes is required

Compilation takes time

No interaction with the browser

We'll cover

  1. The basics

  2. Strategy guide

  3. Pratical Patterns

  4. Project architecture

  5. Tips & Tricks

The Basics

Specs says...

Custom properties define variables

Variables are referenced with var( ) notation

Participate to cascade

The Basics

Tied to selector

Inherited

Dynamic

Live in the browser

:root {
  --link: purple;
}

a {
  color: var(--link);
}

Tied to selector & Inheritance

Simple link

Nav Link

Nav Link

Nav Link

​<a> </a>
<nav>
  ​<a> </a>
  ...
</nav>
:root {
  --link: purple;
}

a {
  color: var(--link);
}

nav a {
  --link: salmon;
}

nav a:hover {
  --link: teal;
}

Dynamic & Live in the browser

Simple link

Nav Link

Nav Link

Nav Link

Cascade
&
Inheritance

Strategy Guide

Dynamic vs Static

 1 

$background: red;
.box {
  background: $background;
}
.box {
  background: red;
}
/* Waiting to compile */

Preprocessors are static

Compiled variables

Preprocessors are static

$background: red;
.box {
  background: $background;
}

$background: blue;
.box--blue {
  background: $background;
}
.box {
  background: red;
}

.box--blue {
  background: blue;
}
/* Waiting to compile */

Update the value of a variable at different points

$background: red;

.box {
  $background: blue;
  background: $background;
}

.box {
  background: $background;
}

.box {
  background: blue;
}

.box {
  background: red;
}
/* Waiting to compile */

Preprocessors are static

Scoped changes

CSS Custom Properties are dynamic

Global vs Local

2

Global Elements

Colors

Brand colors

Spacing

Typography

Global Elements

Colors

Brand colors

Spacing

Typography

Global variables

Identity

Tend to be static

Shared by components

// COLORS
// 
// Base colors
$color-black: #151010;
$color-white: #fff;
$color-gray-light: #eee;
$color-gray: #9e9e9e;
$color-gray-dark: #757575;
$color-primary: #e91f63;
$color-secondary: #00bfa5;
/* COLORS */
/*
/* Base colors */
:root {
  --color-black: #151010;
  --color-white: #fff;
  --color-gray-light: #eee;
  --color-gray: #9e9e9e;
  --color-gray-dark: #757575;
  --color-primary: #e91f63;
  --color-secondary: #00bfa5;
}

Global variables: Preprocessors vs CSS

Local Elements

Re-usable objects

UI Components

Local Elements

Re-usable objects

UI Components

Local variables

Tend to be dynamic

Scoped to the component

Local static variables are OK (sometimes)

.button {
  --button-background: var(--color-primary);
  --button-color: var(--color-white);
  --button-radius: 2rem;
  --button-spacing: 1rem;
  background: var(--button-background);
  color: var(--button-color);
  padding: var(--button-spacing);
  border: 0;
  border-radius: var(--button-radius);
  cursor: pointer;
}

Component Button

// _buttons.scss
//
//  
$button-radius: 2rem;
//
.button {
  --button-background: var(--color-primary);
  --button-color: var(--color-white);
  --button-radius: 2rem;
  --button-spacing: 1rem;
  background: var(--button-background);
  color: var(--button-color);
  padding: var(--button-spacing);
  border: 0;
  border-radius: $button-radius;
  cursor: pointer;
}

CSS custom properties
are local by default 😎

Change the value,
not the variable

3

:root {
  --header-background: blue;
  --header-dark: black;
}

.header {
  background-color: var(--header-background);

  &--inverse{
    background-color: var(--header-dark);
  }
}
.header {
  --header-background: blue;
  background-color: var(--header-background);

  &--inverse{
     --header-background: dark;
  }
}
:root {
   --header-background: blue;  
}

.header {
  background-color: var(--header-background);

  &--inverse{
     --header-background: dark;
  }
}

Change the value, not the variable

Custom Properties and Responsive Design

4

Custom Properties and
Responsive Design

Media query changes

Logic first, design next

// LOGIC

.title {
  --title-size: 1rem;
  @media screen and (min-width: 740px) {
    --title-size: 1.5rem;
  }
  @media screen and (min-width: 1280px) {
    --title-size: 2rem;
  }
}

// DESIGN

.title {
  font-size: var(--title-size);
}

/** LOGIC **/

.title {
  --title-size: 1rem;
}
@media screen and (min-width: 740px) {
  .title {
    --title-size: 1.5rem
  }
}

@media screen and (min-width: 1280px) {
  .title {
    --title-size: 2rem
  }
}

/** DESIGN **/

.title {
  font-size: var(--title-size);
}

Vanilla CSS

The Sass way

Logic first, design next

Practical Patterns

Practical Patterns

Variables

Cascading Values

Scoped Ruleset

Mixins (or multiple values)

Variables

:root {
  --primary-color: orange;
  --background-color: white;
}

body {
  background-color: var(--background-color);
}

header {
  background-color: var(--primary-color);
}

footer {
  background-color: var(--background-color);
}

.button {
  background-color: var(--primary-color);
}

Variables

Define a variable, use it as needed

Cascading Values

Cascading Values

Change the style of an element based on different conditions.

Themes

User preferences

Dark mode

// Variables
// ========================================================================

$subheader-nav-line-height: 6px;
$subheader-height-logo: 40px;
$subheader-animation: all .1s cubic-bezier(.39, .575, .565, 1);

// ========================================================================

.c-subheader {
  border-bottom-width: 1px;
  border-bottom-style: solid;
  border-bottom-color: var(--subheader-border);
  background-color: var(--subheader-background);
  background-image: var(--subheader-background-image);
  background-repeat: no-repeat;
  background-position: center center;
  background-size: var(--subheader-background-size);
  transition: $subheader-animation;

  // Category color variations
  // Default colors are applied to 'notizie' or '/' data-channel

  [data-channel*="notizie"] &,
  [data-channel="/"] & {
    --subheader-border: var(--divider-color);
    --subheader-background: var(--background-color);
    --subheader-text: var(--body-color);
    --subheader-text-hover: var(--hover-02);
  }
  
  @each $category, $name in $categories {
    @if ($category!="notizie"){
      [data-channel*="#{$category}"] & {
        --subheader-border: var(--category-color-base);
        --subheader-background:  var(--category-color-lighter);
        --subheader-text:  var(--category-color-darker);
        --subheader-text-hover:  var(--category-color-base);  
      } 
      @media (prefers-color-scheme: dark) {
        [data-channel*="#{$category}"]:not([data-color-scheme*="light"]) & {
          --subheader-background: var(--category-color-darker);  
          --subheader-text: var(--color-white);
          --subheader-border: var(--category-color-darker);
        }
      }
    }
  }

Scoped Ruleset

Scoped Ruleset 

Scope Custom Properties and use them to streamline our boilerplate CSS.

Links

Buttons

Interactive Elements

Scoped Ruleset

💪 Avoids specificity and inheritance issues
🤟 Separates logic from design
🤙 Works well with components

Scoped ruleset: button 

Mixins

A mixin is a function declared as Custom Property value.

⭐️  Reuse values and logic throughout stylesheets

Mixins

Project Architecture

It's all about the structure

A good structure prevents specificity issues 😎

settings

tools

elements

objects

components

themes

trumps

settings

tools

elements

objects

components

themes

trumps

global settings

mixin, functions, maps

unclassed HTML elements

raw class-based selectors

specific UI objects

theme-based styles

utility classes, states

settings

tools

elements

objects

components

themes

trumps

Global static variables

Global dynamic Custom Properties

 

design tokens

grid gutters

colors

// Global custom properties
//
// Dependencies: 'tools/v7/mq'
//               'settings/v7/spacing.tokens'
// ========================================================================

:root {
  --outer-gutter: #{$gutter-small}; 
  --grid-gutter: #{$gutter-medium};
  --grid-divider-gutter: calc(calc(var(--grid-gutter) / 2) * -1 );
  @include bp(lg) {
    --outer-gutter: #{$gutter-medium};
  }
  @if $font-custom {
    --font-serif-display: #{$system-serif};
    --font-serif-text: #{$system-serif};
    --font-sans-grotesk: #{$system-sans};
  }
}

Settings 

// UI Tokens
// ========================================================================

$ui-01: $color-white;
$ui-01-50: rgba($color-white, .5);
$ui-01-10: rgba($color-white, .1);
$ui-02: $color-black;
$ui-03: $color-gray;
$ui-04: $color-gray-light;
$ui-05: $color-gray-lighter;
$ui-06: $color-gray-dark;
$ui-07: $color-gray-darker;

// Text Tokens
// ========================================================================

$text-01: $color-black;
$text-02: $color-gray;
$text-03: $color-gray-dark;
$text-04: $color-gray-light;
$inverse-01: $color-white;
$inverse-02: $color-offwhite; 

Settings 

settings

tools

elements

objects

components

themes

trumps

Local static variables

Local custom properties

.c-btn {

  // Custom properties
  // ======================================================================
  
  --btn-radius: #{$global-radius};
  --btn-color: #{$ui-02};
  --btn-label: #{$text-01};

  // ======================================================================
  
  padding: $padding-xsmall $padding-base;
  border: none;
  border-radius: var(--btn-radius);
  background-color: var(--btn-color);
  box-shadow: 0 0 0 1px var(--btn-color);
  color: var(--btn-label);
  transition: $global-interactive-transition;

  //
  // States - Disables
  // 

  &:disabled {
    cursor: not-allowed;
  }

Components

settings

tools

elements

objects

components

themes

trumps

Global custom roperties
Local custom properties overrides

// THEME TODAY
// ========================================================================

// Tools
// ========================================================================

@import 'tools/v7/utils';
@import 'tools/v7/colors';
@import 'tools/v7/mq';
@import 'tools/v7/spacing';
@import 'tools/v7/typography';
@import 'tools/v7/zindex';

// Settings
// ========================================================================

@import 'settings/v7/colors.tokens';
@import 'settings/v7/global';
@import 'settings/v7/sizes.map';
@import 'settings/v7/spacing.tokens';
@import 'settings/v7/typography.tokens';
@import 'settings/v7/visibility.map';

// ========================================================================

// Category news colors
 
[data-channel="/notizie/"] {
  --nav-border: #{$category-01};
}

[data-channel="/lifestyle/"] {
  --nav-border: #{$category-02};
}

[data-channel="/cultura/"] {
  --nav-border: #{$category-03};
}

[data-channel="/opinioni/"] {
  --nav-border: #{$category-04};
}

//
// Specific custom properties for Today edtion
//

[data-theme="today"] {

  // Theme Colors list

  --theme-color: #{color( $themes, today, base)};
  --theme-color-light: #{color( $themes, today, light)};
  --theme-color-dark: #{color( $themes, today, dark)};
  --theme-color-darker: #{color( $themes, today, darker)};
  --theme-color-darkest: #{color( $themes, today, darkest)};

  // Category kicker color text

  [data-channel*="notizie"],
  .c-story {
    --story-kicker-text: #{$category-01-dark};
    --story-kicker-bg: #{$category-01-dark};
    @media (prefers-color-scheme: dark) {  
      body:not([data-color-scheme*="light"]) & {    
        --story-kicker-text: #{$category-01};
      }
    }
  }

  &[data-color-scheme*="dark"][data-channel*="notizie"],
  &[data-channel*="video"] .c-story,
  &[data-channel*="foto"] .c-story,
  &[data-color-scheme*="dark"] .c-story,
  &#{$global-background-dark} .c-story,
  &#{$global-background-primary-dark} .c-story {
    --story-kicker-text: #{$category-01};
  }  

  @each $category, $name in $categories {
    @if ($category!="notizie"){
      &[data-channel*="#{$category}"],
      .c-story[data-channel*="#{$category}"], 
      &[data-channel*="#{$category}"] .c-story {
        --story-kicker-text: #{color($categories, $category, dark)};
        --story-kicker-bg: #{color($categories, $category, dark)};
        --story-header-border: #{color($categories, $category, dark)};
        @media (prefers-color-scheme: dark) {
          body:not([data-color-scheme*="light"]) & {    
            --story-kicker-text: #{color($categories, $category, base)};
          }
        }
      }

      &[data-color-scheme*="dark"][data-channel*="#{$category}"],
      &[data-color-scheme*="dark"] .c-story[data-channel*="#{$category}"],
      &[data-color-scheme*="dark"][data-channel*="#{$category}"] .c-story,
      &[data-color-scheme*="dark"] [data-channel*="#{$category}"] .c-story,
      &[data-channel*="#{$category}"] #{$global-background-dark} .c-story,
      .o-bg-primary-dark [data-channel*="#{$category}"].c-story,
      #{$global-background-dark} .c-story[data-channel*="#{$category}"] {
        --story-kicker-text: #{color($categories, $category, base)};
      }
    }
  }

  // Custom properties for each component, light and dark mode version

  .c-brand--resize-medium {
    @include bp(md) {
      --brand-height: 4.5rem;
    }
  }

  .c-menu {
    --menu-background: var(--theme-color);
    @include bp(md) {
      --menu-divider-color: #{rgba($ui-01, .30)};
    }
  }

  &[data-color-scheme*="dark"]{
    @include bp(md) {
      .c-menu {
        --menu-background: var(--theme-color);
      }
    }
  }

  .c-header {
    --header-background: var(--theme-color);
  }

  .c-header__channel {
    --channel-link: #{$ui-01};
  }

  &[data-color-scheme*="dark"]{
    .c-header {
      --header-background: var(--theme-color);
    }
  }

  .c-searchbar {
    @include bp(md) {
      --searchbar-background: var(--theme-color);
    }
  }

  &[data-color-scheme*="dark"]{
    .c-searchbar {
      --searchbar-background: var(--theme-color);
    }
  }

  .c-toolbar {
    --toolbar-border-color: var(--theme-color);
  }

  &[data-color-scheme*="dark"]{
    .c-toolbar {
      --toolbar-border-color: var(--theme-color);
    }
  }

  .c-menu__link {
    --menu-link: #{$link-01};
  }

  .c-navbar__item {
    --item-link-color: #{$link-01};
  }

  .c-navbar__search,
  .c-navbar__controller {
    @include bp(md) {
      color: #{$ui-01};
    }
  }
}

Themes

Tips & Tricks

Using :root { }

Using :root{}

All properties scoped to :root {} in one place / file

Multiple :root {} are OK sometimes but mind the cascade

Avoid code fragmentation

Naming

Naming things is freaking hard

Naming things is freaking hard

/* Naming structure */
--component-property

/* Example */
--header-background
--header-color
--header-background-image
--header-font-size
--header-width
--header-height

Naming things is freaking hard

--button-size
--button-spacing

Custom Properties
and calc() 🎉

Custom Properties and calc() 🎉

:root {
  --spacing: 1rem; 
}

.box {
  padding: calc(var(--spacing) * 2);
}

Custom Properties and calc() 🎉

:root {
  --spacing: 1rem;
  --spacing-small: calc(var(--spacing) / 2);
}

.box {
  padding: var(--spacing);
}

.box--small {
  padding: var(--spacing--small);
}

Negative values

Negative values

Probably get trying this....

.class {
  margin-bottom: -var(--margin);
}
$margin-md: 10px;

.class {
  margin-bottom: -$margin-md;
}

Negative values

..because of this...

:root {
  --outer-gutter: 24px; 
}


// Reset page padding

.o-page-reset {
  margin-top: calc(var(--outer-gutter) * -1);
}

Negative values

...but, you should multiply for -1 🤨

Escaping

Escaping

/* Escape Sass rgba function */
--background: #{rgba(0,0,0, .25)};


/* Escape Sass function + maps */
--background: #{color($categories, $category, darker)};


/* Escape Sass variable */
--background: #{$ui-01};

Fallback (default) values

Fallback (default) values

.c-btn {
  border-radius: var(--btn-radius, 2px);
}

Fallback values

.c-header {
  --header-background: var(--theme-color, #f60);
  background-color: var(--header-background);
}
  

currentColor

currentColor

The currentcolor keyword refers to the value of the color property of an element.

This value was first supported by Opera in 2009, since then, Firefox 3.5+, Chrome 1+, and Safari 4+.

.o-icon {
  vertical-align: middle;
  fill: none;
  stroke-width: 2;
  stroke-linecap: round;
  stroke: currentColor;
  stroke-linejoin: round;
  display: inline-block;
  pointer-events: all;

  &--fill {
    fill: currentColor;
    stroke-width: 0;
  }
}

Final Thoughts

Sass variables

Custom Properties

Change?

It's a custom property

Global static, local dynamic

Thank you! 🙏

@zetareticoli

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francescoimprota.com

CSS Custom properties

By Francesco Improta

CSS Custom properties

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