I am not sure this was the previous behaviour. But now on Rstudio 1.1.383, when switching to a dark theme the scroll bars remain white. I have the RStudio theme set as Modern. Is this the expected behaviour?
Thanks!
I don't think so, my scroll bars are dark. However, my menu bar is white and it looks like yours isn't, so I have no idea what's going on... 
Agree this probably isn't the intended behavior - I'm using Modern with Tomorrow Night '80s as the editor theme and my scrollbars are dark.
Well, the thing is that today, out of the blue, the scroll bars are black again. Puzzling. Anyway, problem solved.
Thanks guys!