Switch to theme compatible with main datahub.io site (?)

[rufuspollock]

As a Docs User I want to feel I am on the same system when I go to the docs so that I feel familiar and that the two things are connected

Evaluation

• Not sure how much work this is so not sure of priority
• Not sure how much value this has

Intuition is that it is low priority especially with datahub-v2/frontend#73