Can I change the background color, e.g. for the fullscreen?

as far as i understood you can do that from within your gtk-theme. look here.

the other option would be to use skins, but i would personally try the gtk-theme way. (having said that, i'm not sure whether the window of the fullscreen is considered a different widget than the normal main window.)

Yes, it can be done with gtk themes, but of course it's not very easy to distribute.
I may try to allow gtk theme settings inside the layout, we will see.

Quote from: ochosi on December 30, 2010, 22:04:39
(having said that, i'm not sure whether the window of the fullscreen is considered a different widget than the normal main window.)
gmb sets the name of the window to the layout name, so it is possible to make a gtk theme that act only on some layouts

You can do that using the skin system, which use a picture, but that can be a 1x1 picture, with this line :
Skin            = background.png::s
(the background.png is first searched in the folder of the layout)

I've just noticed, it seems there is a display bug in some cases when using the Skin option, only part of the window is drawn when switching to fullscreen, it seems to be linked with using the FB container.

I'll probably add a simpler way to do that at some point, I'm not sure how.
note that not all widgets will use the background color, in particular widgets in tabs won't.