Default background now matching the theme light/dark

[andydotxyz]

Abstract image from Jost Grant, thanks!

Checklist:

• [x] Tests included.
• [x] Lint and formatter run with no errors.
• [x] Tests all pass.

[andydotxyz]

Sorry for the wait. This is really close but I think you might want to update the desktop screenshots (in the project root) before merging.

Good point thanks