Rather than send out a bunch of PMs...
To be able to install non-MS signed themes (ie: those that did not fork over cash to Microsoft), follow these instructions:
http://www.neowin.net/forum/index.php?showtopic=140707
Next, follow these instructions to create an appropriate place to install themes:
http://www.themedoctor.com/er15c.shtml
Now, you need to get some themse.
www.themexp.org has themes in the standard .theme format, so go there for themes. Place them into your C:\Program Files\Plus!\Themes folder, making sure that the .theme file is in that folder and other files are in a subfolder. Now, you can use these themes simply by using the Display Properties dialogue. You might have to browse to the folder to get the themes, but that should be easy.