Changed buttons to use background colored borders, instead of margins (same as horizontal buttons)

Signed-off-by: Martin Evans <martindevans@gmail.com>
Added fillers for left and right elbows (an element which sits underneath content and fills in empty space).

Signed-off-by: Martin Evans <martindevans@gmail.com>
Merge pull request #28 from martindevans/feature/elbow-content

Feature/elbow content