Swapped out some values for sass variables

You can find all of these defined here: https://github.com/alphagov/govuk_frontend_toolkit/tree/master/stylesheets. They're really useful <:^]
Removed superfluous div, reworded message.

That div didn't need to be there so I yanked it.
I also reworded the message a bit so the link doesn't have to be in brackets.