Improve theme definition

[bugadani]

This PR reimplements DefaultTheme in a bit more flexible way, and allows extending the theme with additional color schemes.

Closes #55 and maybe others

Remaining:

• CheckBox
• RadioButton
• Slider
• ScrollBar
• Only define a scrollbar style once (H, V)
• Define multiple states with the same styling (StateA, StateB { ... })
• Relax static str requirement in button-like objects
• Label styles (e.g. Header)
• Use in examples
• Move checkbox/radio rendering to theme
• Add border and display clear color to theme (border as a widget?)
• showcase the title widget

After impl, decide if helper macros should/could be deduplicated.
Investigate how to allow more color options in RGB color schemes.
Define styles required to close #23 and #36, #37

[bugadani]

I'm still not quite happy with where this goes. Ideally, I would like to set a primary and seconday accent color, and the theme should generate all colors necessary. This will probably be the focus of a future PR