Basic theming support #97
Description
It was suggested somewhere that Bosca Ceoil should have a way to customize its looks, at least to some degree. I don't think a complete redesign via a custom theme is something feasible, at least this time. Bosca's style is very important and also helps us design UI widgets in a particular way.
But what can be made adjustable is the color palette. Most elements are drawn using plain colors and that can certainly be exposed to the user. One tricky part that immediately comes to mind is the logo, which exists in several different colors, corresponding to the instrument color palette. So this needs to be resolved somehow, so custom colors are respected there. The rest should be relatively straightforward to do.