Hi
The buttons are held in 5 png files in the images directory. They are named for the foreground color of the button like - ui-icons_d404e7_256x240.png
These are created for you by Theme Roller when pick the icon color in the clickable active,default and hover sections as well as the error sections.
You can open them in an editor and change the colors (don't rename the files), but I just let Theme roller do it.
HTH
Chris