Discourse is incredibly customizable!
The goal of this topic is to show you how make use of all the amazing options that are available to you as a theme developer. You know… so that you don’t have to reinvent the wheel 
Variables? What variables?
Variables cover a large number of things, from font sizes to colors to z-index values. You can use and override the vast majority of variables in your theme editor.
Colors
There are many colors available for you to use or override in your theme. You can see the basic set of colors in the admin UI, under Customize > Colors. Here is a screenshot of the default color scheme:
These colors are available as CSS custom properties that you can use and override via this syntax:
var(--primary)
var(--secondary)
var(--tertiary)
var(--quaternary)
var(--header_background)
var(--header_primary)
var(--highlight)
var(--danger)
var(--success)
var(--love)
The use of SCSS color variables (like
$primary, $secondary` and so on) is deprecated. Discourse core uses CSS custom properties since August 2020 and we can recommend all existing themes switch SCSS variables to CSS custom properties. For complex color manipulations, see this guide.
So how can I use them?
Start by creating a new theme or if you are using a git-based theme, create a new theme component and add it to your theme.
Let’s say I want to set the <body> background to match the highlight color of the current color scheme. Well, I can do that like so:
body {
background: var(--highlight);
}
And the color would be pulled automatically from the current color scheme, like so:
(This background color is not recommended 
)
Now, let’s say the current color scheme is not set to “Light scheme” for the current theme. Instead, let’s say it’s set to the default “Dark” scheme, the same code would yield a different result because the variable is different.
You can see how the <body> background follows the current highlight variable whatever it may be for the current color scheme.
That covers the basic color variables. Discourse also uses derivative colors that can be used or overriden. You can see the full list of colors from the current color scheme in your browser inspector, here is a screenshot of the default theme:
Notice that you can inspect these colors on any Discourse instance, no need to be authenticated. On your own Discourse instance, you can (and should) also use the styleguide plugin, which is now included in core. Enable it and head to /styleguide, and you see all of the Discourse UI elements, including colors:
(That’s the Grey Ember color scheme above.)
Ok, on to more advanced topics:
Advanced Variables
Take a look at this file in the Discourse repo:
This is where most of the variables are defined. Just like colors, these variables are magically available for you to use in theme stylesheets.
I will try to break down the content of the file below:
Widths
$small-width: 800px !default;
$medium-width: 995px !default;
$large-width: 1110px !default;
these are very handy if you want your theme to match native Discourse behavior, especially in media queries for example take a look at the relevant parts of the pre-compiled scss for .user-info:
.user-info {
&.medium {
flex: 0 0 32%;
margin: 0 2% 4vh 0;
@media screen and (max-width: $small-width) {
flex: 0 0 48%;
margin-right: 0;
}
}
}
Notice how the media query is fed a variable to determine a cut-off point for small screens and apply different styles.
Fonts
--base-font-size: 0.938em; // eq. to 15px
--base-font-size-larger: 1.063em; // eq. to 17px
--base-font-size-largest: 1.118em; // eq. to 19px
// Font-size definitions, multiplier ^ (step / interval)
--font-up-6: 2.296em;
--font-up-5: 2em;
--font-up-4: 1.7511em;
--font-up-3: 1.5157em;
--font-up-2: 1.3195em;
--font-up-1: 1.1487em; // 2^(1/5)
--font-0: 1em;
--font-down-1: 0.8706em; // 2^(-1/5)
--font-down-2: 0.7579em; // Smallest size we use based on the 1em base
--font-down-3: 0.6599em;
--font-down-4: 0.5745em;
--font-down-5: 0.5em;
--font-down-6: 0.4355em;
This is also pretty straightforward, Discourse has an awesome font-scaling system and so you should make use of it.
The units used are in em and so would be relative to --base-font-size which is used on the <html> element.
So:
.btn {
font-size: var(--font-up-6);
}
gets compiled into:
(–base-font-size * --font-up-6) or (14px * 2.296) = 32.14px
Or this:
Line heights
--line-height-small: 1;
--line-height-medium: 1.2; // Headings or large text
--line-height-large: 1.4; // Normal or small text
These variables can be used just like the previous example. Do note that they are unitless values. Here’s what that means:
Z-index values
$z-layers: (
"max": 9999,
"fullscreen": 1700,
"modal": (
"tooltip": 1600,
"popover": 1500,
"dropdown": 1400,
"content": 1300,
"overlay": 1200,
),
"mobile-composer": 1100,
"header": 1000,
"tooltip": 600,
"composer": (
"popover": 500,
"content": 400,
),
"dropdown": 300,
"usercard": 200,
"timeline": 100,
"base": 1
);
These can be used as discussed here:
div {
z-index: z("header");
}
“or when nested, like this:”
div {
z-index: z("modal", "dropdown");
}
“You can also do basic math as you’d expect:”
div {
z-index: z("base") + 1;
}
The code-blocks above are part of the quote but I moved them out for formatting
Now here’s an example:
Put a box above the header:
.zbox {
position: fixed;
top: 0;
left: 0;
height: 200px;
width: 200px;
background: red;
z-index: z("header") + 1;
}
Put a box below the header:
.zbox {
position: fixed;
top: 0;
left: 0;
height: 200px;
width: 200px;
background: red;
z-index: z("header") - 1;
}
Box shadows
$box-shadow: (
"modal": 0 8px 60px rgba(0, 0, 0, 0.6),
"composer": 0 -1px 40px rgba(0, 0, 0, 0.12),
"menu-panel": 0 6px 14px rgba(0, 0, 0, 0.15),
"card": 0 4px 14px rgba(0, 0, 0, 0.15),
"dropdown": 0 2px 3px 0 rgba(0, 0, 0, 0.2),
"header": 0 2px 4px -1px rgba(0, 0, 0, 0.25),
"kbd": (0 2px 0 rgba(0, 0, 0, 0.2), 0 0 0 1px dark-light-choose(#fff, #000) inset),
"focus": 0 0 6px 0 $tertiary,
"focus-danger": 0 0 6px 0 $danger
);
As you can sort of see, most of the work has been done for you when it comes to variables, and you only need to utilize what’s already there in most cases.
The use for box shadows looks like this:
.d-header {
box-shadow: shadow("focus");
}
This should make the header use the focus box-shadow. And if we check to confirm:
This post is a wiki so edit as needed 