Desktop: Moved all themes out into their own .js theme files (#2951)

* Working on an aritim-dark theme, very WIP still

* Update aritim.js

* Added new theme "Aritim-Dark"

* removing aritim-dark "brace" theme for now

* Updating style with some changes

* Reset branch back to the original upstream state, to create a different pull-request for moving all themes outside

* Reset branch back to the original upstream state

* Moved all themes out into their own .js theme files

* Updating master to use the seperated themes stuff

* Moved themes into their own const

* Removed switch case for themes and simplified it. Thanks Laurent for the help!