The theme toggle has three states. How do you model this with a checkbox?
(Also, technically, alternative stylesheets can be defined in HTML, except every browser except Firefox removed it: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/...)
Good question, especially since the Ruby site already does this by default. Perhaps the argument is that one of the two color schemes may be designed so poorly that the user may want to manually switch to the other one.
I agree that using radios would be better. Or just prefers-color-scheme, which sidesteps the FOUT issue that often occurs when storing theme settings in localStorage.
