I just want to say kudos to the author of this work. It is something I considered attempting myself a number of years ago.
I think the comments about generating msstyles files rather that patching uxtheme has merrit. Could it be dome as a tool that generates the theme (and caches it), and then hook that into the wine boot-up via a registry startup key?
The other thing I observed is that many gtk themes do odd things that could break your conversion process - as Ruslan observed. There will be many edge cases, so a rich set of tests would be helpful. It may be necessary to put in some perceptual algorithms to iron all the glitches out - though if you're lick that won't be necessary.
Anyway, this is really exciting work and I look forward to it going out to a really wide audience.
Joel