in skin studio open the section xp taskbar there you will find
the start panel.On the left in the explorer section you will
find the different sections that the start panel is comprised
of.Find the parts you want to change and import your new graphics
Be sure you have the correct margins and sizes or you'll drive
yourself crazy trying to get what you want!
good luck

Sounds like it may be a more complicated task than you are prepared to undertake. You would need to convert the .msstyles file to WB format, then edit the section mimiibelle refers to above in SkinStudio. Obviously, you would need WindowBlinds installed to make it all work. SkinStudio can't edit .msstyles files directly. To avoid potential conflicts, you would need to uninstall StyleXP before installing WindowBlinds. There might be a simpler way out there to accomplish what you want, but I'm not aware of one.