Updated header and main body styling

Added yoast_breadcrumb to the header
Recoloured the green elements of the header and body
Removed support for custom backgrounds
Revised support for custom headers
Added z-index 99 to search toggle button
Added Font Awesome support
Added Javascript to make the nav menu sticky and add the even class to every
other row of any table on the site
Fixed general positioning of the whole site to suit the design