cs_themes: Add a simplified UI (#11548)

* cs_themes: Add a simplified UI

* UI: Padding adjustments

* Only show "Custom" in the combobox when needed

* Show the custom page if the system uses a custom set of themes

* Support default modes and variants

* Simplify code

* Refresh combo as part of refresh
Load themes earlier to be able to access them from anywhere in
the code.

* Refactor code to use Classes

* Refactor theme loading

* Check if themes are installed

* l10n: Go back to original msgids

* Remove legacy/dark sorting

Joseph doesn't like it ;)

It doesn't work well with some of the themes which don't follow
our naming convention.

* Refresh custom page before showing it

* Tune msgid

* Add HighContrast to styles

* Add HiDPI support

* Make dots slightly larger

Author: clefebvre