Support custom themes

[mherrmann]

It's already possible to change font size and face. This should be extended to make it possible to customise the visual appearance much more. The most important thing to change are obviously all colours.

(In implementing this feature, don't forget to make it possible to style the highlight colour of selected items in GoTo on Steroids. One user specifically asked for this.)

[pslobo]

A possible way forward would be to reuse ST themes. At least the specific UI parts that make sense for fman.

[mherrmann]

That's a very interesting idea!