Using CSS variables¹ to ship customizable CSS in Django apps

I have been working with SASS for a long time but have been moving towards writing CSS with a few PostCSS goodies in the last years. At first, I just replaced the $... with var(--...) and didn’t think much about it. The realization that CSS variables can be more than that came later. Edit basic values directly in the browser and immediately see the results! Change CSS depending on media queries or the cascade!

With all that power came back the wish to not just ship backend and HTML code in Django apps I (help) maintain but also reusable CSS, with a few overrideable CSS variables for basic changes to the visual style. Loading .scss files from somewhere inside venv/lib/python3.11/site-packages/<package>/styles/ would of course have been possible, but very obscure. Also, not everyone puts their virtualenv at venv, the README instructions for those packages would quickly have become unwieldy. CSS variables paved the way for shipping CSS as a Django static file while still allowing customizability by leveraging the functionality of the browser itself instead of the frontend build toolchain.

Patterns for overrideable values

A pattern for defining defaults for CSS variables is to always define the fallback (the example is intentionally bad but inspired by real world experiences when developing feincms3-cookiecontrol):

.box {
  background: var(--box-background, #222);
  color: var(--box-foreground, #fff);
}

Less repetition (but trouble awaits)

If --box-background isn’t set the var() function falls back to the second argument, #222. Repeating this value over and over gets annoying quickly, so you define a few defaults on the :root element and use those variables in the code, without specifying the default again:

:root {
  --box-background: #222;
  --box-foreground: #fff;
}

.box {
  background: var(--box-background);
  color: var(--box-foreground);
}

The project can now override the default background color using:

:root {
  --box-background: #444;
  --box-foreground: #ccc;
}

Of course now you’re back at the mercy of CSS loading order. If the app’s CSS is loaded first, everything works. If not, your custom value is immediately overwritten. You could avoid this by overwriting the default lower in the cascade:

.box {
  --box-background: #444;
  --box-foreground: #ccc;
}

Great, everything works again!

Later, the box also contains a button which uses a different background but the same foreground, so of course we add more variables in the package:

:root {
  --box-background: #222;
  --box-foreground: #fff;
  --box-button-background: #333;
  --box-button-foreground: var(--box-foreground);
}

What happens now when overwriting the --box-foreground variable just for the .box element?

You’re not sure? I certainly wasn’t and am not. But what I remember happening was that the overridden foreground color was just applied to the text and not to the button itself. I was confused (it seems clearer in hindsight…)

A better way

If values are supposed to be overridden and only used inside components, a better way is to define local CSS for components by following a convention (underscore prefix for local/private variables):

/* Defined on .box, not :root */
.box {
  --_background: var(--box-background, #222);
  --_foreground: var(--box-foreground, #fff);
  --_button-background: var(--box-button-background, #333);
  --_button-foreground: var(--box-button-foreground, var(--_foreground));
}

And then you only use the prefixed versions inside the component:

.box {
  background: var(--_background);
  color: var(--_foreground);
}

.box__button {
  background: var(--_button-background);
  color: var(--_button-foreground);
}

The --box-* variables are undefined by default; the only time when they are set is when the user of the package wants to override those values. If you only overide the box foreground the button also inherits the new foreground color. And while there would certainly be a way to achieve the same thing with the old way above it’s certainly not as simple to explain.

The reason why it’s simple to explain is immutability. The CSS variables which are overrideable by the user are only ever read by the package, never written to.