We're having a new house built, and the kitchen backsplash that was just installed is a tumbled stone, which looks great, but isn't a flat surface. As a result, the wall plates for electrical outlets, switches and phone jacks don't sit quite flush. Is there something that can be done about this, to fill the gaps with some sort of caulking or something, or is this just something I have to accept?
There are insulating gaskets for switches and outlets like these
They would help seal against air leaks, but they might not visually fill the gaps at the edge of a switch plate.
There are also deep beveled switch plates, such as those found here. You could custom grind these down with a dremel-type tool to fit the highs and lows of the surface, but that would be a pretty time consuming task.
While you probably could caulk the gaps, you should be careful to avoid getting any into the box and wiring itself. Also, caulking will make removal of the cover plate a bit more difficult.
CAULK IT!!! Work like an artist not a contractor. DON'T "smush caulk everywhere then wonder why it looks like butt when finished. Use ONLY what you need and "tool it in/smooth it out" using a damp finger, wiping frequently. I have been known to mask off the rectangle around the wall plates prior to starting (duct tape works very well on stone). I recommend trying to find Polyseamseal in your stone's matching color. There are also many colors to choose from (not Polyseamseal but still good stuff) back by the tile and grout section of H-D & Lowes. I just remembered another job I did that resulted in miracle stuff. I used clear caulking (NOT sillycone or butyl!!! Use latex/acrylic only). Again, I masked off rectangle + added a second coat of clear caulk to compensate for initial shrinkage, then I painted the caulking with EXACT matching paint which I mixed (you will only need a Tablespoon) using one of my daughter's artist brushes.
You could work out what the gaskets that are made of that were linked in @bib's answer.
It may be possible to purchase similar material from a place like McMaster-Carr that you could cut larger than the cover plate. Then install the cover plates with the material in place. After installed use a very sharp razor blade type knife to trip the excess material away using the edge of the cover plate as a guide. Be careful to not gouge into your back splash material.
Another idea that comes to mind...if you are using white cover plates, is that you could go to the hardware store and get a couple of rolls of the self stick type weather strip material that is about a half inch wide. This could be adhered to the back of the cover plate so that it is projecting out beyond the cover plate by some amount. The try installing the cover plate and see if you can trim off the excess like I described above.
Either of these schemes may not work very well at all if the cover plates are too flexible.