Anderton
I would say it's simple, but daunting. IOW the actual ways to change the colors is easy, but actually deciding which elements to change, how they change, and how deep you want to go, gets you into the weeds pretty fast.
Some things are easy to change, like background colors. Others require working with an image editor. Creating a new theme from scratch is not for the faint of heart, but making a few tweaks isn't hard if you know how to use an image editing program.
Yes, but...it's "little" things like colour matching and font sizes and gradients if you want to edit an existing item or build from scratch.
For example, I really like the dark of Tungsten, but the 3D of Mercury; I'd like to use a hybrid of the two (much like Lord Tim's done, but with "real" buttons), by using the Mercury icons where I can in (which is really easy to do via the Theme Editor, but the way, as you can "remove" a custom change and revert to the Mercury element) - sadly, that makes the borders of the icons and shading not match the Tungsten background, so the replaced icons look out of place when mixed in (I spent about an hour editing *just* the Track Inspector buttons to get them to work and I'm bored!).
Even just changing a background colour on an existing icon isn't easy as they often have gradients.
That said, the end results if you are prepared to spend the time, can be good and what you want. I will see about sharing my theme later when I'm happy(ish) with it.