In reply to Jami Gibbs.

@Jami that's exactly right - the point of the Cutomizer is to create a unified experience for customizing the site, with a live preview of changes before publishing them. For many, many users, selecting and changing themes is the most important part of the site customization process, hence this project.

I'll also point out for everyone that while the Customizer controls window is fairly small, this is a balance with providing a reasonably sized preview of the front-end, and the narrow controls UI window is mobile-first out-of-the-box. Being forced to work with less real estate in the Customizer controls forces developers to simplify their UIs and make things easier to use. If you're shoving hundreds of options into the Customizer, you're creating something that's just as bad of an experience to use as if you'd done that in a custom admin screen.