The buttons and submit buttons in Powerstore uses a global classes that are defined int he common.css file. If you open the common.css file look for these two classes:
input[type=submit] and input[type=button]. These two classes control most of the buttons in PowerStore. You can then add background images to customize those buttons.
Once you make this modification, i think Theme-it will be a great tool that will help you customize the color of the buttons and the background images of the buttons. But currently you Theme-it will only help you in changing the colors.