A Dark Mode Toggle Without JavaScript

Have you ever seen a toggle on a website that allows you to switch between a dark and light mode? Now it’s possible to implement one without JavaScript, thanks to :has() CSS selector.

:has() premise is simple. Select a parent that has a given selector. Be it children or sibling using + or ~. With Firefox stable adding support for :has() last month, the browser support reached 90% according to caniuse.com!

Without further ado, let’s start with HTML for the toggle:

<!-- ... -->
    <input id="dark" type="checkbox">
    Dark mode
<!-- ... -->

I hope you see where this is going! Now, the CSS for this is packed. Packed with tiny bits and tricks I learned. These are: declaring variables on elements for an override, using :has(), and using :checked state selector.

body {
  --fg: black;
  --bg: white;
  color: var(--fg);
  background-color: var(--bg);

body:has(#dark:checked) {
  --fg: white;
  --bg: black;

Here is a live example using the above code (and a border):

There are a couple more things that we can do such as:

I have created an example page with these improvements. You can view it at tymekdev.github.io/no-js-dark-mode-toggle and its source code on GitHub.