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))
My view is much the same as usual really - it'll essentially always be that we want to use our standard font, currently Benton. But adding the flexibility to override this is potentially useful for when used on other products. I'd be happy if it was fixed to Benton as there's little real-world likelihood anything else would be used, but more than happy to have the flexibility to change if it can be introduced with no overhead and won't add complexity or likelihood in defaulting to the wrong font when used.

Just to clarify Alberto, these decisions will never be left up to the product developers per se, it'll be the designers - we can call it the 'product' as an overall description, but the product developers don't work on design. Small semantic point, but worth mentioning :)