I'm no master themer or anything but I've made quite a few framework mods for the X and made a theme for Apex
most of what I learned to do was by using google and just kind of playing around. I would suggest getting 7zip and using root explorer and making a COPY of your framework-res.apk, place the copy on your sd card so you can move it to your computer. use 7zip to get to the res\drawable-hdpi folder and take a look in there, you will find a lot of the images that you would want to change to make a "simple theme"
hopefully some of the info was useful for you.