(excuse delayed reply-) I would only work on a skin if it was possible in the old fashioned way. Plus's skinning doesn't offer the level of control I really like to have; like mouseover and animation effects (which appeared in the 2011 version anyway). I think MS wised up a bit and did well to allow a generous amount of control over the look of the client, but there's still the charm of using something sleek and skinned, something I miss.
It is actually possible to use realllly old clients by hexediting file version numbers. But I end up missing just a handful of the newer features (like animated display pics) which aren't possible in the old clients. I've also discarded all the resources I had for all my skinning, so I'd be re-learning and remaking anything from scratch, almost.
The basic idea is to use a resource editor/hacker to extract images and layout information. make changes you like, then add them back. Hopefully you get what you want to work. Any previous experience with making windows visual styles is great, as they are similar in many ways. It takes a lot of knowledge, patience, and experimentation.
Well, if a .msstyles file is the core of a visual style, then the WLM equivalent is msgsres.dll. since version 7 I think, they moved all the UI files to there. The formatting of all the UI elements uses the same code as the text files found in the .msstyles files. But not all features/tags are interchangeable... e.g you won't be able to get text glowing with a specified color like the Window titles. But many things can be made to slide, change, and fade with mouseover images in much the same way.
and I don't specifically remember how to do those.. I'd need to study again to offer any more advice. There will be plenty of resources on the net.
Those were good times indeed.. I feel less motivated to do wlm skinning, the ability to customize 'out of the box' has really improved in new versions. And, endless other projects are begging for attention.