Custom docs theme

[jwallwork23]

To be honest, I don't really like the theme that we are currently using ("nature"), I just thought it was the best of the templates that I saw.

It would be great to have a custom theme like the Firedrake and Thetis websites do, perhaps with a banner at the top that uses an interesting adapted mesh.

[ddundo]

The current one is also terrible on the phone

[jwallwork23]

It'd be nice to use a theme that uses syntax highlighting for code.

[ddundo]

Could probably use one of the top ones in https://github.com/topics/sphinx-theme :)

[jwallwork23]

We discussed that it'd be nice to use the same theme as Firedrake/Thetis, although we'd need a banner image with the right aspect ratio. Perhaps lock exchange?