So first of all, Get ResEdit and open your msstyle with that application 
Go to butmap section and use the arrow keys to go down while you look at the bitmap previews on right,
There will be some bitmaps similer to
Frame Caption
Frame Minimised
Frame Maximised
You can edit those bitmaps as you want and also can replace them with new ones 
If you're not capable of modifying them, then, go to the Temp folder in the ResEdit directory, where you installed the programm (C>Program Files>TGTsoft>ResEdit) and copy the files (the bitmaps you want to change) to another location
Then attach them here and I'll look to modify them 