Creating CSS Global Variables : Stylesheet theme management [duplicate]

Latest Update: 16/01/2020

CSS Custom Properties (Variables) have arrived!

It's 2020 and time to officially roll out this feature in your new applications.

Preprocessor "NOT" required!

There is a lot of repetition in CSS. A single color may be used in several places.

For some CSS declarations, it is possible to declare this higher in the cascade and let CSS inheritance solve this problem naturally.

For non-trivial projects, this is not always possible. By declaring a variable on the :root pseudo-element, a CSS author can halt some instances of repetition by using the variable.

How it works

Set your variable at the top of your stylesheet:

CSS

Create a root class:

:root {
}

Create variables (-- [String] : [value])

:root {
  --red: #b00;
  --blue: #00b;
  --fullwidth: 100%;
}

Set your variables anywhere in your CSS document:

h1 {
  color: var(--red);
}
#MyText {
  color: var(--blue);
  width: var(--fullwidth);
}

BROWSER SUPPORT / COMPATIBILITY

See caniuse.com for current compatability.

Firefox: Version 31+ (Enabled by default)

Supported since 2014 (Leading the way as usual.)

More info from Mozilla

Chrome: Version 49+ (Enabled by default).

Supported since 2016

Safari/IOS Safari: Version 9.1/9.3 (Enabled by default).

Supported since 2016

Opera: Version 39+ (Enabled by default).

Supported since 2016

Android: Version 52+ (Enabled by default).

Supported since 2016

Edge: Version 15+ (Enabled by default).

Supported since 2017

CSS Custom Properties landed in Windows Insider Preview build 14986

IE: When pigs fly.

It's time to finally let this ship sink. No one enjoyed riding her anyway. ☺

W3C SPEC

Full specification for upcoming CSS variables

Read more

TRY IT OUT

A fiddle and snippet are attached below for testing:

(It will only work with supported browsers.)

DEMO FIDDLE

:root {
  --red: #b00;
  --blue: #4679bd;
  --grey: #ddd;
  --W200: 200px;
  --Lft: left;
}
.Bx1,
.Bx2,
.Bx3,
.Bx4 {
  float: var(--Lft);
  width: var(--W200);
  height: var(--W200);
  margin: 10px;
  padding: 10px;
  border: 1px solid var(--red);
}
.Bx1 {
  color: var(--red);
  background: var(--grey);
}
.Bx2 {
  color: var(--grey);
  background: black;
}
.Bx3 {
  color: var(--grey);
  background: var(--blue);
}
.Bx4 {
  color: var(--grey);
  background: var(--red);
}

<p>If you see four square boxes then variables are working as expected.</p>

<div class="Bx1">I should be red text on grey background.</div>
<div class="Bx2">I should be grey text on black background.</div>
<div class="Bx3">I should be grey text on blue background.</div>
<div class="Bx4">I should be grey text on red background.</div>

You can't create variables in CSS right now. If you want this sort of functionality you will need to use a CSS preprocessor like SASS or LESS. Here are your styles as they would appear in SASS:

$Color1:#fff;
$Color2:#b00;
$Color3:#050;

h1 {
    color:$Color1;
    background:$Color2;
}

They also allow you to do other (awesome) things like nesting selectors:

#some-id {
    color:red;

    &:hover {
        cursor:pointer;
    }
}

This would compile to:

#some-id { color:red; }
#some-id:hover { cursor:pointer; }

Check out the official SASS tutorial for setup instructions and more on syntax/features. Personally I use a Visual Studio extension called Web Workbench by Mindscape for easy developing, there are a lot of plugins for other IDEs as well.

Update

As of July/August 2014, Firefox has implemented the draft spec for CSS variables, here is the syntax:

:root {
  --main-color: #06c;
  --accent-color: #006;
}
/* The rest of the CSS file */
#foo h1 {
  color: var(--main-color);
}