I wanted to customize the default theme on the brand spankin’ new 1.5 so I made a little psd file with a few layers that made it quite easy to change the look quickly. Just stay within the guides and you should be just fine. Be sure to backup or duplicate your default theme before you start. It is a photoshop file with layers that correspond to the different images needed for the theme. The Guides also give a visual feedback for where each slice will start.
The layers are named to correspond with the exported images. I just added new layers with my look and left the original ones alone. Then I could flip back and forth by toggling layer visibility.
When you have finished just use save for web with all the slices and images only and you should be set.
There will be one extra file (spacer) for slice 06 but it can be ignored. Upload these 5 files into the images folder inside the theme you are changing. Voila. You’re done. I know everyone doesn’t have Photoshop but that is what works for me. The file is quite simple. Sometimes the best way to figure something out is to give it a try.
Update: There is a new file with added layers and masks. More Info