You might be surprised how little there is to know. There are basically three strategies: 1. set your light theme color variables in :root and your dark theme colors in `@media (prefers-color-scheme: dark) { :root { ... } }` 2. use the newish `--var: light-dark(light-value, dark-value)` syntax with `:root { color-scheme: light dark; }` 3. use a class on the body to determine whether to apply light or dark variables (can be used in combination with the above to default to user’s current theme while letting javascript toggle the theme by toggling the class)

If you're just concerned about light vs dark, then this article (which was posted to HN) does an auto/light/dark toggle button without javascript, and it shows using CSS variables for your colors: https://lyra.horse/blog/2025/08/you-dont-need-js/