Light- and dark mode CSS, the easy way

One of the never-ending tech-battles on social media right now is the battle between light- and dark mode. Everyone knows this is ultimately a matter of good and evil. Light side of the force vs. dark side of the force.

Whether you prefer light mode or dark mode: as a web developer, there is a high chance you need to know how to support both modes. When building the Schedule-X calendar, I used a method for supporting both modes with very low effort. And here's how you can do it too.

Step 1: define all colors as CSS custom properties

Firstly, you need to define all the colors you want to use in CSS custom properties. You can create a color.scss file, looking something like this:

:root {
  --color-primary: #6750a4;
  --color-on-primary: #fff;
  --color-primary-container: #eaddff;
  --color-on-primary-container: #21005e;
  --color-secondary: #625b71;
  // ... and so on
}

Now, if you're using SCSS like I do, you might be tempted to write these colors as SCSS variables, like $color-primary. But don't! These SCSS variables only exist at compile-time, and won't be around to be altered at run-time where we will need them later.

Step 2: using the CSS custom property-colors everywhere in your application

If you want to successfully use this method, you need to stick consistently to this rule. Wherever you use colors in the application: use them through CSS custom properties. Even if it's "just" a simple border color, don't cheat; border colors typically need a very different nuance in dark mode than they do in light mode! So for example:

.my-rule {
  border: 1px solid var(--color-neutral);
}

Step 3: Create a single dark-mode rule

In the same file where you defined your colors for light mode, you can add a rule for the selector .is-dark, like this:

.is-dark {
  --color-primary: #d0bcff;
  --color-on-primary: #371e73;
  --color-primary-container: #4f378b;
  --color-on-primary-container: #eaddff;
  --color-secondary: #ccc2dc;
  // + other properties
}

Now what does this rule do?

Basically, it just means, that whenever you slap the class name is-dark onto an element, the dark mode colors will start being applied to everything inside of that element - as long as you consistently used the custom properties for defining color.

And that's it! Creating a dark mode for your web project does not have to be hard.

If you want to play around with the concept yourself, here's a brief demo for how it works on Codepen: