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 
`theme.scss` just imports other files. The CSS that affects the theme is in those other files, such as `main/_base.scss`; it's in those files that you should make the CSS changes you need to make.