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.

The normal panel window.

.gtkrc-2.0

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"

Class of the hidden panel window.

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

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

Class of the images used on most of the panel plugins.

.gtkrc-2.0

style "xfce-panel-image-style"
{
  # Fix the panel images to a default gtk icon size, is only works for
  # images below 32 pixels. You can for example use this when you have
  # fuzzy launcher icons.
  XfcePanelImage::force-gtk-icon-sizes = true
}
class "XfcePanelImage" style "xfce-panel-image-style"