Doc: Graphviz fontcolor switched between light and dark themes

[Thalley]

Is your enhancement proposal related to a problem? Please describe.
When generating figures using graphviz, the fontcolor stays the same when switching between light and dark theme. Since it does not switch, all graphviz will have to rely on a fillcolor like white.

Describe the solution you'd like
The color of of the font in graphviz should follow the same color as the rest of the documentation, unless specifically specified by the figure.

Describe alternatives you've considered
N/A

Additional context

[kartben]

Will probably give astropy/sphinx-automodapi#167 (comment) a try

[kartben]

See #86509 -- now looks like: