Right now, the themes you can pick from the menu are simple CSS files in the themes/ directory. Create an easy theme maker to allow anyone to create these files.
Essentially, the design should be a simple dialog box with two sections:
1. A bunch of SVG elements
2. A set of sliders and dropdowns for configuring color and font style
Clicking on any element in the first section should allow you to configure it's style using the controls in section 2.