Should we have an "arviz theme"?

[OriolAbril]

In sphinx, it is possible to define themes that extend other themes and set up some variables/extra css only which keeps them quite minimal. One example would be: https://github.com/matplotlib/mpl-sphinx-theme.

We already use the theme in python.arviz.org and in https://preliz.readthedocs.io/en/latest/. If we did something like this we could also update https://einstats.python.arviz.org/en/latest/ to use this theme.

That theme would set our own website color schemes and custom css so we don't need to copy them everywhere, some custom shared elements like the donate button, I think the navbar icons too, things like that.