[Improvement] Access all installed themes in Firefox et al.

[noyannus]

Include all installed themes in Firefox (Librewolf, etc.), similar to #158. Currently there are only four basic options.

The theme names can be found in the zipped and renamed extensions in

~/.mozilla/firefox[/Profiles]/<profile>/extensions/
~/.librewolf[/Profiles]/<profile>/extensions/

There, it's [<mailto>|{UUID}].xpi —> rename *.zip & extract —> ./manifest.json; line 5 ( "theme":...) can be used to filter for theme; line 4 has its name: "name": "<name>".

Haven't tried if zgrep is usable here.