Add settings screen
@nicol Basic settings screen using mixins to auto-do most of it.
Added two settings:
- CMS menubar: Copied from theisland. theisland loads additional css file so added the code but not the css.
- Dark mode: Setting is not used yet.. Looking at the code I guess we'd need to split the existing css so the "@media dark" stuff is in a separate css file and can be conditionally loaded (or replace standard css in the case of "force dark mode".