768px-979px Screen Header fix + incl. older fixes

The last few changes (the 15px fix and container padding) have been
moved to structure.css. I also fixed a bug where the header bar would
cover up part of the screen on resolutions between 768px and 979px.
Merge pull request #144 from Pure-Battlefield/bugfix-15px-misalignment

768px-979px Screen Header fix + incl. older fixes