Slider thumb's background cannot be styled

[grizeldi]

I'm trying to write a theme. I copy-pasted the button commands from the default glass style and added a background change, since I have a different image for pressed and unpressed buttons. For most buttons this works just fine, slider's thumb button, however, just loses the background completely as soon as I press the button (and change the background). If I comment out the background change on button down, it disappears as soon as I release the button (when the next background change happens).

A couple people on JME discord reported having the same issue.

[pspeed42]

Looking through issues I never noticed before because gmail doesn't sort them nicely. Do you have some sample code or something so I can see what's going on? Thanks.