.MSSTYLES File Extension
What is an MSSTYLES file?
Theme file for Windows XP that contains colors and images used by windows, buttons, and other elements within the Windows GUI; two popular MSSTYLES from Microsoft include the Luna and Zune themes; the Royale theme is used by Windows XP Media Center Edition.
MSSTYLES files must be digitally signed by Microsoft in order to work correctly with the Windows operating system. Custom themes developed by users can be implemented using the StyleXP program.