Dimmer Switch Install - Trying to Locate Ground
It's the bundle of bare wires in the rear right of the box
Since we are dealing with NM cables here, we know we need to look for a bundle of bare wires in order to find our grounding connection.  As it turns out, there is a bundle of bare wires heading through the center of the box and into a wirenut in the back right; it's simply not obvious that they're bare (I can tell from the diameter) because someone slopped a bunch of white paint and/or drywall mud into the box during construction.