Allow the user to swap between curated themes

Discussed this with Greg Watson. Preferably, developers are just laying out the components needed for their application and the theming code makes it look good by default. There will always be some tweaking of whitespace usage by developers, but use of color and good component defaults should be things they never need to think about.

He also mentioned wanting a second theme besides the one I'm currently using in the app launcher that is inspired by an old-school QT application, with the hope that someone who is using an older application we want to replace will be more comfortable.

With all of this in mind, this moves away from the developer choosing theming and gives the user a choice of two themes: 1. the current app launcher theme and 2. a "technical" theme that tries to vaguely mimic the Fusion theme here: https://doc.qt.io/qt-6/gallery.html. Both of these will need to be modified over time, but that's going to be hard to do without user feedback so I wanted to put something quick together that can serve as a proof-of-concept.

Closes #2 (closed)