Os provided theme support

[ptsavol]

Add a setting to select an appication theme provided by a third-party package.

Fixes #1952

Checklist before merging

• [ ] Documentation is up-to-date
• [ ] Release notes have been updated
• [ ] Unit tests have been added/updated accordingly
• [ ] Code has been formatted by black
• [ ] Unit tests pass

[trobob]

This update would be much appreciated by all the vampires out there with sensitive eyes. Dark mode woud be especially nice for the DB editor which is currently all white and a little hard on the eyes after a while.