I've written an application using Python and Glade. The app has got a toolbar with a dark background, and each toolbar button has got a custom image (i.e. no stock icons are used).
I've noticed that when using stock icons instead of custom images as icons, whenever the mouse hovers over the button the button is highlighted, which does not happen with my custom images.
Thunderbird seems to go a step further and rather than highlighting the icons, it inverts their color, which works quite well for a dark theme (see below):
Button not highlighted, normal color
Button highlighted on hover, color inverted
How can I get my custom icon images or the buttons to be highlighted on hover?
The issue is a problem with the Ambiance theme. There is a highlight, but the background color for the prelight is darker. If thunderbird is showing a light highlight and inverting the colors of the icons, then it is because it's using custom widgets in XUL (which isn't Gtk+), which aren't following the GTK+ or icon themes, for everything.