Colour changes to better  adopt Docker IO styling guide.

 - Changed bg to a light shade of gray.
 - Panels are now white
 - Some highlights and headers are orange
 - Breadcrumb home is a home matching the main nav. menu

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Merge pull request #5197 from ostezer/mkdocs-legendary-colour-updates

Colour changes to adopt Docker IO styling guide.