My advice would be to go to the 'official' Xfce links.
They could better answer your questions about customizing the Xfce Panel.
Xfce/Wiki pages >> http://wiki.xfce.org/
Xfce Website >> http://www.xfce.org/
Xfce FAQ >> http://wiki.xfce.org/faq
Xfce Forum >> http://forum.xfce.org
I know from personal experience, that the panels color can be changed to practically any color you want, to include a 'true' transparent panel.
And you can add themes to the panel . . . but, you'd have to checkout the Xfce Wiki and Forum.
You might, also, checkout this person's blog about what they did with the Xfce desktop and the Xfce Panel.http://blogs.gentoo.org/nightmorph/2010 ... ce-desktop