Overriding :root CSS variables from inner scopes

Question

In our design system at Stack Overflow, we use Less to compile CSS color values.

We have global Less variables like @orange-500 that are frequently modified for hover states, building border styling, background colors, etc.

In Less, this is written as darken(@orange-500, 5%). I'm trying to achieve something similar using native CSS variables. Switching to CSS variables will allow us to ship features that rely on theming (Stack Exchange Network, Dark Mode, etc.) much faster, with way fewer lines of CSS, while enabling swapping variables on media query (high contrast, dark mode, etc).

This example of overriding our color’s lightness value in hsl works when the variables are scoped to a CSS class:

.card {
  --orange: hsl(255, 72%, var(--lightness, 68%));
  background: var(--orange);
}
.card:hover {
  --lightness: 45%;
}

<div class="card">
  Hello world
</div>

However, we need to specify our color variables globally in a single, swappable place to support global theming, but this doesn't work as expected:

:root {
  --orange: hsl(255, 72%, var(--lightness, 68%));
}
.card {
  background: var(--orange);
}
.card:hover {
  --lightness: 45%;
}

<div class="card">
  Hello world
</div>

I've tried switching from :root to html or body without any luck. Any workarounds to this?

Answer 1

This is a scoping issue. The way you're doing it, you're inheriting --orange from the :root, and --orange in the :root has a lightness of 68%.

In order to change it, you'll want to re-scope the --orange variable to an element that will look up the new --lightness value. There's a few ways to pull this off:

Option 1: duplicate the --orange variable on the element:

:root {
  --lightness: 68%;
  --orange: hsl(255, 72%, var(--lightness));
}
.card {
  background: var(--orange);
  --orange: hsl(255, 72%, var(--lightness));
}
.card:hover {

  --lightness: 45%;
}

<div class="card">
  Hello world
</div>

Obviously this kinda stinks, because you're going to have to duplicate that --orange variable.

Option 2: You could abstract the other parameters of --orange so that it's not as duplicative. I'd be a fan of this approach despite the fact that it's more text:

:root {
  --lightness: 68%;
  --orangeHue: 255;
  --orangeSat: 72%;
  --orange: hsl(var(--orangeHue), var(--orangeSat), var(--lightness));
}
.card {
  background: var(--orange);
  --orange: hsl(var(--orangeHue), var(--orangeSat), var(--lightness));
}
.card:hover {

  --lightness: 45%;
}

<div class="card">
  Hello world
</div>

What you could also do is scope this specifically to a .darkMode class that might be applied to the HTML element or the body. This could also make sense because it's clear what the intent is from the code:

Option 3

:root {
  --lightness: 68%;
  --orangeHue: 255;
  --orangeSat: 72%;
  --orange: hsl(var(--orangeHue), var(--orangeSat), var(--lightness));
}

.card {
  background: var(--orange);

}
.card:hover {
  --lightness: 45%;
}
.darkMode .card {
  --orange: hsl(var(--orangeHue), var(--orangeSat), var(--lightness));
}

  <div class="darkMode">
    <div class="card">
      Hello world
    </div>
  </div>

Regardless of how you go, the issue is that the --orange variable is inheriting from its original scope where --lightness is set. Think of it as "inheriting a computed value".

In order to get --orange to get the new lightness, you need a new --orange somewhere.

Option 4

I'm not sure what your theme pattern is, but I can explain how I created a dark mode on my own blog . If you look at the CSS What you'll see is that I've created two complete themes that follow the same naming convention:

--themeLightTextColor: rgb(55, 55, 55);
--themeLightBGColor: rgb(255, 255, 255);
--themeLightAccentColor: rgb(248, 248, 248);
--themeLightTrimColor: rgb(238, 238, 238);
--themeDarkTextColor: rgb(220, 220, 220);
--themeDarkBGColor: rgb(23, 23, 23);
--themeDarkAccentColor: rgb(55, 55, 55);
--themeDarkTrimColor: rgb(40, 40, 40);

What I then do is create a third set of variables whose job it is to be the "active" managers:

--themeActiveLinkColor: var(--linkColor);
--themeActiveLinkColorHover: var(--linkColorHover);
--themeActiveTextColor: var(--themeLightTextColor);
--themeActiveEditorialTextColor: var(--themeLightPltNLow);
--themeActiveBGColor: var(--themeLightBGColor);
--themeActiveAccentColor: var(--themeLightAccentColor);
--themeActiveTrimColor: var(--themeLightTrimColor);

Then, I scope the active theme settings under a single class:

.theme--dark {
   --themeActiveTextColor: var(--themeDarkTextColor);
   --themeActiveEditorialTextColor: var(--themeDarkPltNLow);
   --themeActiveBGColor: var(--themeDarkBGColor);
   --themeActiveAccentColor: var(--themeDarkAccentColor);
   --themeActiveTrimColor: var(--themeDarkTrimColor);
}

It seems like maybe your intent is to not have to explicitly declare a theme, but rather tweak some "root variables" to adjust it. But I would suggest that maybe you have a pattern in place where a single class can change an active theme. The advantage to this pattern is that you would be able to also adjust any "root variables" on the class name.