V2 theme refactor

Most notable changes:

- Deprecate color use cases
- Add anchors to demos for better testing
- Rewrite button generation pattern /cc @AlbertoElias there's less magic happening in there. Let me know what you think!

This PR also fixes many visual regressions introduced in the previous merge.

Idea for a next step: improve demos to cover more cases (elements, [backgrounds](http://jsbin.com/gezize/1/edit?html,output))
To be clear, I would prefer to dictate the font here. I just assumed (as often is) that there was an "ideological" code reason why the font isn't prescribed. @danskinner what do you think?