Sometimes, when I download a theme it is pretty good, but not quite right. A simple way you can fix this is to edit your .gtkrc-2.0 file in your home directory. It seems to me, though, that a lot of the options in this file could easily be pulled out into a nice gui. This could include dropdown lists of available options, color choosers, file selectors, and other niceties that would make simple edits to the desktop environment a much more pleasant task.
For an example of how this could look, I was thinking of something along the lines of Glade, where you have a list of widgets on the left, a list of options for the selected widget on the right, and an example window in the middle.