.MSSTYLE File Extension
What is an MSSTYLE file?
Theme or "style" that can be applied to the Windows XP interface; includes custom images, colors, icons, and other elements; allows users to change the overall look and feel of the Windows desktop.
NOTE: MSSTYLE files are similar to .MSSTYLES files, but are not digitally signed. Since Windows XP only supports signed themes, the uxtheme.dll, shsvcs.dll, and themeui.dll files must be patched in order to use the unsigned themes.