html repr doesn't work in some sphinx themes

[dcherian]

Downstream issue: xarray-contrib/cf-xarray#57
Example: no reprs displayed in https://cf-xarray.readthedocs.io/en/latest/examples/introduction.html

@benbovy's diagnosis:

It looks like bootstrap 4 (used by sphinx-book-theme) forces all html elements with hidden attributes to be actually hidden (source), so the hack in #4053 does not work here (the result is even worse).
I guess that a workaround would be to add some custom CSS such as .xr-wrap { display: block !important }, assuming that custom CSS is loaded after Bootstrap's CSS. Not ideal, though, it looks like a hack on top of another hack.

[dcherian]

This should be fixed: executablebooks/sphinx-book-theme#238 pydata/pydata-sphinx-theme#274 pydata/pydata-sphinx-theme#275