I've been toying a little with LXPanel theming because I'm trying to improve the overall experience in my distro (openSUSE) that is currently... let's say not very good. I've discovered that it is very easy to customize LXPanel by modifying an existing GTK2 theme or by creating a minimal one just to skin the panel. Examples:
I have zero artistic talent, but they're only for demonstration purposes, XD. Well, what I'd like to have is an option to choose a custom gtkrc to theme the panel. I "think" that it is very similar to choosing the current System theme option, but I may be completely wrong. If this could be implemented, bingo!, you have almost full theming support for LXPanel.
What I do now is just a hack: I create a new gtk theme, make the gtk-3.0 folder a symlink to the system theme and create the gtkrc inside the gtk-2.0 folder (adding an include "system-theme" at the beginning) just with the modifications for the panel. Given that LXPanel won't be ported to gtk3 (AFAIK) we only have to worry about gtk2 stuff.