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 
You can change the content of any file in the repository as long as you do it in your fork. When you PR it back, whoever reviews it will tell you whether or not your change is appropriate.

In this case, as you're making changes to the WET theme and that's where the theme CSS is stored, yes, that's where you should make the changes in those files.