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.

.gtkrc-2.0

XfcePanelWindow
---------------
style "xfce-panel-window-style"
{
  # Time in miliseconds before the panel will unhide on an enter event
  XfcePanelWindow::popup-delay = 225
 
  # Time in miliseconds before the panel will hide on a leave event
  XfcePanelWindow::popdown-delay = 350
 
  # Size of autohide window in pixels
  XfcePanelWindow::autohide-size = 3
}
class "XfcePanelWindow" style "xfce-panel-window-style"