Customisation of theme’s radio styles in Settings

[JayPanoz]

Right now to build the Theme radio in settings, the component takes the background and text colors defined in their Preferences’ object, and applies these as inline styles instead of exposing everything as custom variables on :root.

While this is fine at the moment, the styling of themes’ settings in UIs can vary quite dramatically, so it would be great to make this more easily styleable – circles w/o text but an accessible label, background with a template text, etc.

Note auto is a special case with its own logic, and may be well the one developers would want to actually change. Right now it’s a linear gradient from the light theme background to the dark theme background.