On and off, I've been toying with the idea of patching the Uxtheme library in memory, similar to what others do and unfortunately charge for. Well, I've finally gotten off my bum and put together a working client/library for use by users and developers alike. Despite the prefix (lib), the code can be compiled into a usable < 100kb client executable that, upon execution, loads the Display Properties applet and patches Uxtheme memory in the background. For the duration of the Windows session, you can experience custom themes. No down payment. No critical file replacement.
I'm hoping someone sees the light and uses the library to create a theme manager of sorts, something I do not have the time for. The theme manager would have to show available themes, let users apply them, and employ the use of a service to execute the patching code (or force everyone to be an administrator, eek).
I'm also hoping some kind soul helps better the code and sends me recommendations/changes. I'm eager to learn (and fix) what I royally screwed up.
Have fun, remember it is GPL'ed.
Download The Code.