I recently switched to XFCE in light of the fact that Gnome2 and Metacity are being deprecated. I've managed to arrange things the way I like it, but have found one severe limitation in configurability. Unlike with Metacity, it appears to be impossible to change the colors of window tops, sliders, and so on. I dug through the theme directories to figure out why. The answer is that the various window decorations are made up of image files. The colors are embedded in those files. I don't know if this was done for speed or what, but it probably makes it impossible to change the colors without a complete rewrite of how themes are created.
Is there any work being done to allow for color tweaks of the sort I'm looking for?