Added links back to top-level page from tags and projects.

Trying to fix #7.
Make large blocks more colourful.

Large block elements (header, jumbotron) were previously shades of
grey, but are now more "OSM-like" colours. This is the first step
towards making the site more visually appealing, as pointed out in
issue #8.
Add 'screen' qualifier for local override CSS.