DOC: Change color scheme of documentation to match bioscan-browser

[scottclowe]

Adjust the default readthedocs theme so the colours match @annavik's style guide for bioscan-browser.

Before:

After:

Thanks to @annavik for proposing this, and refining the stylesheet to use.

[scottclowe]

I'll merge this in and we can make any further updates if necessary in a new PR.

[scottclowe]

Actually, before I merge it in, you can check out the build here: https://bioscan-dataset.readthedocs.io/en/doc_new-color-scheme/

[annavik]

Thanks for your patience on this Scott! I had a look, I think it's looking good 👌

I pushed 2 small updates:

• Tweak source link color (to match other links)
• Update version select appearance (to give it some better contrast and similar style to other buttons)

From my side, I think we can merge this now!

[scottclowe]

Great, thanks Anna! Agreed - I also thought the contrast was too low on select when it was rendered. I'll merge it in now.