- reworked core themes
- reduced explicit colors by using color functions
- made it easier to create new themes
- avoid using colors from one component type for another component type
(except when useful; e.g. `Separator.foreground` is also used for other separators)
- HelpButton: use colors from Button (instead of from CheckBox.icon)
- ToolBar: improved "docking" and "floating" colors
- In "Flat Light" theme, changed styles of focused and default buttons to
avoid confusion with all other themes. Focused buttons now have a white
background (was light blue) and a slightly wider border. The default button
now has a light blue background (was white) and a thin border. In all other
themes the default button also has colored background.
- In "Flat Dark" theme, use slightly wider border for focused buttons.
- CheckBox and RadioButton: In "Flat Dark" theme, use blueish background for
focused components.