changing background colours on buttons (semi-permanently)

[Dirk Koopman <djk tobit co uk>]

I have a stateful dialog box that requires the user to click their
desired button and then click another one (OK) to exit. I would like the
desired button to change its background colour (to dark green for some
obscure historical reason) to indicate that it is selected (think *huge*
radio buttons here) whilst waiting for OK or cancel to be pressed. 

Associated with this question is the fact that: whilst I seem to be able
to change the font and apply it in a style, none of the fg[xxx] or
bg[xxx] in the same style seem to do anything. There is obviously a
theme of some kind running (it is ubuntu) but it seems difficult to
override (or find out what it even is).

Dirk