In order to support both a theme toggle and to automatically use the
user's selected theme based on browser/system defaults the stylesheets
has to be duplicated twice since the latter requires the css to be
wrapped around a media query.
This duplication is a painful experience to deal with when adding or
changing existing styles. Even more so when it involves jumping around
the behemoth default.css from and back to whatever sections you were
just working on.
I don't believe the we the Invidious team will be able to agree to a
proper solution anytime soon (eg css post-processor, modern css
light-dark feature, etc) so this commit is here as a stopgap measure.
The workaround is to move the theming styles to two separate files
which are read at runtime and used to generate a combined stylesheet
with the necessary duplication for the media query. This combined
stylesheet is then delivered on a new route added to Invidious,
bypassing the static file handler.
