Some initial information about theming the panel and different applets will go here. This is currently very incomplete. If you have any questions you can ask in #gnome-art on GIMPnet.
The panel has a PanelToplevel class that can be used to theme only the panel. This will only work for in-process applets though (and I am too lazy right now to look up what can be done for other applets).
For example, this should work to theme any menu bar inside the panel (ie. window list and menu bar applet):
widget_class "PanelToplevel.*.GtkMenuBar" style "panel-menubar"
The only way to draw a background is to use a bg_pixmap in the gtkrc.
Main Menu and Menu Bar
The Menu Bar Applet is a normal ../GtkMenuBar that has a shadow of NONE. One can check the NONE shadow to make it transparent. The same applies to the Window Selector
Theming the menus that are opened
With gnome-panel 2.24 it will be possible to theme the menus that are opened by the Menu Bar Applet and the Main Menu Applet. The menus are named "gnome-panel-main-menu", so they can be themed with a match similar to the following:
# This will theme the menu, and all widgets inside. widget "GtkWindow.gnome-panel-main-menu*" style "gnome-panel-main-menu"
The context menu of the panel is called "gnome-panel-context-menu". This is only the context menu that can be used to configure the context menu of the panel, not the one of applets. (It is more of a by product of adding the "gnome-panel-main-menu" name.)
The Window Selector is a GtkMenuBar with a shadow of NONE, similar to the Menu Bar Applet.
The window list has some code to glow applications that need attention. This glowing can be controlled with some style properties. (TODO: update ../StyleProperties and copy the relevant things here.)