Make the 'contents' links in the homepage header legible during :focus state

Focus state applies an orange outline/background, which the white link text had terrible contrast against.

This is a fix to a single instance of a wider problem, but it's the most high profile case i've found so far.
Work to fix the wider :focus contrast issue is ongoing, but i'd rather we fix this in the mean time
Merge pull request #594 from alphagov/homepage-link-focus-legibility

Make the 'contents' links in the homepage header legible during :focus state