Add an option to change the highlighting background color of `updatemenus`

Currently only the normal (i. e. unhighlighted) background color can be changed through the parameter bgcolor. If you set this background to a dark coloring, you most probaby also want to adjust the text font to a bright coloring so that the text is still readable. But then the text becomes pretty unreadable while being highlighted since the highlighting background color is very bright!

Therefore I think an additional parameter bgcolor_active or the like should be introduced.

Additionally, an even more sophisticated solution could involve an automatic adjustment of the text font color and/or background color depending on the color brightness of the user defined bgcolor and/or font.color values (this seems to be already the case with the font color of hoverinfo labels).

[jsandiego]

I’d like to bump this issue. Would be great to have this customization!

[vidasci]

I would also love to see this being implemented, plotly plots with a dark theme look great but it’s a big downside having a bright background in updatemenus (and being unable to change it)