You can also swap out the CSS file for one that explicitly sets/unsets light/dark mode. This allows for a website that works in any browser even if your Javascript breaks or doesn't load.

Or you could use cookies. No need to mess with localStorage. Just set a cookie "prefersDarkMode" to true or false and have the backend send the right file. No need for an interactive backend, nginx can handle serving files based on cookie contents. No need for cookie popups/walls either.