MSSTYLES is a Microsoft file format, that contains the bitmaps and metadata for the Windows XP skinning engine, first introduced in Windows Whistler Build 2250.
The engine, in its unmodified state, only fully applies .msstyles files that have been digitally signed by Microsoft, such as Luna or the Zune theme. The default Windows XP style is known as Luna, but additional custom-made styles are available on the Internet – however, few are digitally signed. Four other signed styles include Royale (Media Center Edition) (Energy Blue), Royale Noir, Windows Embedded Standard CTP Refresh, and the Zune Style.
Custom-made styles can be used using different methods, by means of manual replacement and automation by executable files. Many popular and freely distributed theme-patching executable files can be found online.
Windows Vista and Windows 7 also use .msstyles files for skinning (like the Aero.msstyles file), however the format of these files is completely different, so .msstyles files are not transferable between Windows Vista and Windows XP. This version of .msstyles file contains PNG images and metadata.
There is a manual way to replace three DLL files (uxtheme.dll, shsvcs.dll and themeui.dll) to use unsigned custom visual styles in Windows Vista.