Styles: Gtk4 prep

[danirabbit]

In latest GTK 4, the only non-deprecated use of StyleContext is to set a provider for the whole screen. Since we have a finite number of monitor colors, we really don't need to generate this CSS. Instead we can just have a set number of color style classes and then assign a style class to each display widget.

• Set the CSS name of display widgets
• Create color-number classes with predefined styles
• Tiny style stuff, make the label slightly bolder, give the widget a little rounded corner, clear icon palette to improve contrast of star icons
• Add one style provider to the screen, not to each widget for each color

[danirabbit]

@jeremypw sorry about that! Fixed the typos and cleared the warnings. Dunno how I missed that :grimacing: