Add new Getting Started page

Modified content in start-en.hbs to match the new theme, 
Added page specific css in content/_base,scss
Modified colours in main/_base.scss and footer/_base.scss
Modified link colours in _variables.scss 
I'd suggest using something like <code>page.src</code> instead of <code>page.filename</code> so you can be more specific (including part of the directory structure) so you don't risk accidentally including something else that may have "start" somewhere in the filename.

A less messy approach could be to add a temporary font-matter variable to the pages that you are changing (e.g., "newdesign": "true") so you can use that variable to target just the pages you are touching. Later on you can remove that variable when the changes are global.