Improves the Theme Sandbox in developers plugin

I was working on adding layouts to the theme sandbox and ended up fixing/improving the sandbox. This adds a very simple theme for the sandbox so that the site theme does not break the sandbox (or is at least less likely to do that). One change is making the position of the sidebar static so that you can always jump to another page without having to scroll back up.

![screenshot from 2013-07-17 07 55 01](https://f.cloud.github.com/assets/199558/811519/2269bbf8-eed8-11e2-8f2c-1c3111b10ca2.png)