The .msstyles skinning method died basically because no one developed an easy to use program to skin the .msstyles format. Sure there are hacks out there to modify the required system files to allow the applying of non-digitally signed .msstyles files for XP, Vista and Windows 7 (and even one that only modifies the files in memory called AVE Styler), but no one ever developed an easy way to actually design .msstyles skins after TGTSoft died. One attempt was made with a product called AVE StyleBuilder, but to tell you the truth it would take a rocket scientist to try and figure out how to use it. I myself developed many .msstyles for Windows XP over the years, but had to give it up when support for it died. Stardock is the only company to develop a fairly easy to use product to design and skin all three OS's with ease. Granted SkinStudio could still use a lot of improvement, but it is the best thing out there today for designing OS skins, and Windowblinds itself has seen extreme improvement of late, as have many of their products.
So you probably should not expect much support from software developers for created skins in the .msstyles format. With the death of TGTSoft I do not believe there are any visionaries left.