Revamp "Theme" settings page

[Studio384]

Luna will no longer allow users to choose their own theme. No worries, through, themes will be able to provide additional color schemes if they want to. However, for our new Theme Engine, we will have to make some serious changes to the Backstage's Theme-section.

The new Theme page on the Backstage will show the current default theme's info box on the left hand side, while on the right hand side, it can display theme options. Below those 2 panels, you can find all other installed themes.