Most all your themes can be customized by changing you index.css file and your images:
So you'll need to learn how to work CSS and PhotoShop or Fireworks or any other comparable software.
Now to create you own theme you will need learn how to work PHP-5/HTML XHTML coding along with the other stuff and now we are moving on toward the HTML5 and CSS3 for the near future and most main browsers in use today!
I'll try to place some help help when ever I get the chance and Lesmond and I along with some of our good friends work with customizing themes, how ever we are working and looking toward the future of themes and graphics WP type and mobile units view of the web browsing. I'm also working on trying to eliminate most graphics in the themes or as little as possible....just to keep up.
Of course hobby sites will always welcome heavy graphics, but business and professional site are looks for fast and easy!
More later >>>>