Yes, it is possible. I do that all the time to themes that insist on using system colours in some dialog boxes or make text in some windows hard to read. You need an archive handler that can open the archive and extract and install files back into it. 7zip is such an application. The files you would be editing are stylesheets mostly, with an extension name of .css. Of course, you can also make changes to images or add images and change the code to use them. You can also keep a theme installable by editing the install.rdf file to increase the number of the max version listed for the target application.