Hmmm, I posted an answer yesterday which has disappeared.
It was not a solution for your problem. I suspect that you want to use the same color as project background color and the problem is to see the border of the slides in that case? In that case I could propose a workaround. Set up guides in a contrasting color as slide border. Here is the screenshot I created yesterday to show what I mean. The project has the default resolution of 1024x627, and is non-responsive. If you are on responsive projects it is even easier since the rulers in such a project are in %
