More seamless background images for larger displays

- Made the 4 main hero images subtly fade to black
- Make body background colour black to hide background edges
- Slightly adjusted related background styles to avoid !important
declarations where possible
Remove unnecessary empty body rule
Actually adding updated background images

Ooops.
Merge remote-tracking branch 'upstream/master' into background-images
Merge pull request #611 from WillsB3/background-images

Background images