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.