First step, which you shouldn’t skip. DUPLICATE THE DEFAULT TEMPLATE. Don’t work on the original, in fact don’t even make the default template writable. By making a duplicate, you always have something to easily fall back on or compare.
As for the actual templates, you’re going to have dive in and get your hands dirty
. Like EE, it seems overwhelming at first, but it DOES make sense and there is a logic to the design. It just gets a little insane with the shades of purple and different link styles. Try to change colors at first, so you figure out the structure and then you can try and get more fancy.
You can go through the CSS template and figure out the colors via the hex numbers. I did this by cutting and pasting the hex numbers into Photoshop. Make a photshop document with all these colors in one document, so you can see the color palette as designed. Then you can assign your own colors and the various shades.
When changing the actual graphic files, I kept the original graphic on separate layer and used that as a model to change it to what I wanted it to look like.