Comment 9 for bug 284975

If you select a theme which includes a defined panel bg in the gtkrc it will change. If you do not select a theme in which a panel bg is defined you need to right click on the panel, select the Properties menu entry, select the background tab and then select the "None (use system theme)" option.

The default theme was set this way to avoid a bug in the windows spawned by the panel (about, for instance) in which the panel bg is used as a background for the entire window.