I've (slightly) modified the existing theme. I don't know how, and I don't know when, but at some point during the development of my site, I borked it for Internet Explorer (you can check it out yourself here. I installed a second copy of Propulsion and, sure enough, it renders just fine in that blasted IE.
So now I need to figure out what's causing the borkage.
Anyone have any suggestions on PC software that can scan two folders for line changes in files? Any other ideas aside from copying one file, checking the site, copying another file, checking the site?