This works without JavaScript, and starts out with the user preference
(according to the browser) by default. If JavaScript is available, the
toggle writes to LocalStorage.
Instead of having the link to the homepage (via the logo) and the theme
switcher in the footer, split this up. The logo is now in the top left
and the theme toggle in the bottom right. The footer itself is removed
on the homepage, but kept for blog posts, guides, etc.
Lorenz Leutgeb