Feature: Appearance settings > Dark Mode/Window Color

[blawnode]

Feature request:

• Add an additional color to the Colors>Item selector for the window's currently white-gray background color.

Implementation checklist:

• Color option to selector UI
• Implement in theme/color scheme export/import
• The behavior itself

(Issue formatted like #284 due to similarities)

[blawnode]

If the issue is applicable, there's likely a need for the same tags as #284 : enhancement, tracker-ui.