A few weeks back I posted a fairly hacky solution to have theme aware icon buttons (like the ones on the application bar). At the time, I knew there must be a better way. I’ve found it now.
If you download the set of standard Windows Phone 7 icons, you’ll notice there are two sets: one for dark backgrounds, and one for light backgrounds. The built-in application bar magically picks the right icon, but if we want to use these icons for our own buttons, we need to switch between them.