This allows users to set/customize an instance's theme and appearance to
their liking. The config CSS field is prepopulated with all default CSS
variable values to allow quick editing.
Note that this can be somewhat of a "footgun" if someone updates the
CSS to hide all fields/search/etc. Should probably add some sort of
bandaid "admin" feature for public instances to employ until the whole
cookie/session issue is investigated further.
* Implemented new dark theme
Now uses a dedicated css file for all dark theme color changes, rather
than replacing color codes directly.
Color theme is from discussion in #60.
* Minor link color update
