Change the theme color for the docs

[wcarthur]

Would prefer to use GA standard colours for the documentation, but seems the sphinx_rtd_theme is fairly stubborn about allowing this sort of change.

The primary colours are (all in hexadecimal representation):
"#006983" - Deep teal
"#A33F1F" - Deep red/brown - for the BAL this is the prefered option as the main colour
Highlights: "#72C7E7" - sky blue
Secondary colours:
"#A5D867" - Pale green
"#CA7700" - Orange
"#6E7645"- Green/grey
"#988642" - Dark tan
"#003145" - Deep blue
"#5E6A71" - Grey/blue