WIP Themes: Initial Light Theme Support

[Foldex]

Initial support for a light adwaita theme, can serve as a base for other light themes. Mostly works, though various places need fixing and polish:

Needs work

• [ ] Settings
• [ ] Game Overlay
• [ ] Friends
• [ ] Game Details Manage/Details/Favorite Buttons
• [ ] Game properties DLC/Workshop Headers
• [ ] Library sidebar game name hover
• [ ] Library sidebar filter dialog
• [ ] Steam Loading Throbber (dunno if this is fixable)

Preview

Associated Issue

#212