Allow the user to swap between curated themes

Discussed this with Greg Watson, but basically rather than making the developer choose which theme they would want to use for a particular app (potentially yielding very different experiences across the many applications), it would be better if we provide a set of curated themes that the user can choose between. I think for the first iteration, we will provide two themes that the user can swap between:

1. Technical - Web 1.0, minimal use of color, inspired by QT
2. Modern - Web 2.0, heavier use of color, generally flatter, minimalistic design

The initial implementation really shouldn't flesh these themes out completely, as that is going to be a long-term, iterative process.