[FR] Retrieve Plot Attributes

[nickbrady]

It would be great if Plots had ways to retrieve plot attributes, not just set them. For instance it would be useful to setup conditional control protocols depending on what theme or background color is being employed.

i.e.

display(Plots.theme)
> :dark

[BeastyBlacksmith]

I startet implementing this in #4783, but put it on hold because of CI issues.