[DHPersonal]

This is really cool and a great example of the upgrades to CSS. The benefit of JavaScript would be setting a state in localStorage to remember the light/dark value that was set, as yours does not remember what was set after a shift+f5 refresh of the site.

[TymekDev]

With JavaScript you could make it a three state toggle: light, dark, and auto. The last one would allow to to back to the default - no preference.

I thought this might use indeterminate checkbox, but that wouldn't be necessary. 'Light', 'dark', and lack of thereof in localStorage already amount to three distinct states.