Besize the appearance settings in the panel preferences, there are also a couple of classes you can style through the .gtkrc-2.0 file. Below we'll summarize the global classes. Some plugins also have gtkrc style properties for specific settings, you can find those in the manual page of the specific plugin.

XfcePanelWindow

The normal panel window.

XfcePanelWindowHidden

Class of the hidden panel window.

XfcePanelWindowExternal

The class of the external window used for the 4.6 plugins.

XfcePanelWindowWrapper

The class of the window used for external plugins running in the wrapper application.