Themes are now controlled with a class on the body element.
If a preference is set the body element will have either "dark-theme"
or "light-theme" class. If no preference is set or the preference is
empty the class will be "no-theme".
"dark-theme" and "light-theme" are handled by darktheme.css and
lighttheme.css respectively.
"no-theme" is handled by default.css where depending on the value of
"prefers-color-scheme" the styles corresponding to "dark-theme" or
"light-theme" are applied.
Unfortunately this means that both themes are duplicated, once in the
theme .css and once in default.css.
