The greatest extension in terms of theme(s) creation at the most humanistic end-user level.
Also if it's possible can you make every possible piece of the browser color palate customizable? For example you label "Tool Bar Icons and Text" can you further break it down where for example one arrow key is one color, and the other a different. Or both arrow keys the same color, but then the drop down menu a different color. Also if you can make it mobile friendly, considering the market is now mobile. I would greatly appreciate it.
Basically what I'm getting at is I love how easy you've made it but I wonder if it can be even easier in terms of more selection. The same way you guys designed the "Customize..." droplet that allows you to put what you want on your GUI bar for quick link access. In regards to individual paint, because honestly it's so cool what you can do with such a simple template.
I feel that will bring a lot more end users if they can completely customize the coloring. People love cosmetics, and even further implement moving images, etc.