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?