Some sites (and something I did while refactoring my portfolio) is levarage that to CSS. You can set is as to read the light/dark mode setting from the OS and provide a "toggle" to let the user choose between light and dark. This doesn't even need JS, just a couple hidden `<input type="radio">`s and some bits of CSS.

Although as I've said in another comment, things like this, "go to the top" buttons, cookie banners and the like should be task of the web browser, not of the website.