Intersphinx: differentiate tooltips with content comming from other projects

[humitos]

It would be good to have a clear visual differentiation between a tooltip showing content from the same project we are reading from a tooltip with content from another project connected via intersphinx.

In #86 we are adding an extra class to these nodes (intersphinx) that we can use to differentiate them in some way. What would be a good visual differentiation?

Reference: https://github.com/readthedocs/sphinx-hoverxref/pull/86#discussion_r635527788