We have walnut cabinets, a dark grey floor and stainless steel counters - very similar to your setup, but we have white painted boxes. We went with a handmade recycled tile, the debris series from fireclay. It was a fight to the death between a pale blue and a pale sage green, and a lot of discussion was had about format - 2x8 versus 4x4, etc. They have a lot of colors and formats to choose from. I was inclined toward white until it was all installed, and I realized that was too boring and the room needed color.

It doesn't matter that much. In fact, it my be better if the sill went in first so the tile at the jambs (if there is any) could die on top of it. Have the sill installer leave the returned ends proud of the wall enough that the tiler can get thinset, tile, and a 1/16" gap for silicone behind them. This is where trade coordination is critical. I'd have the tiler dab hot melt on a small piece of blocking on the wall that's the thickness of the thinset, tile, and gap. Sill man butts the return ends to the blocking and the tile man knocks out the blocking when he lays the tile. Sill man templated to the blocking; no way to screw up.

there are a zillion versions of each approach available in the Photos section of this site -- just search for "quartz counter and quartz backsplash" or "quartz counter and tile backsplash" and start bookmarking the ones you like. eventually you'll get a sense of what fits your taste.

I’m currently underway with my kitchen reno, and ordered the Viatera Quartz in Soprano for my countertops. Like many, I’m struggling with style & color of backsplash. I’m doing a 36” fireclay farmhouse sink in biscuit (not white), so wanting a milky, creamy backsplash to complement and pick up the undertones in the counter, possibly herringbone (but don’t want too busy or competing). Faucet is Moen Weymouth in brushed gold & cabinets will be painted a soft lighter gray (possibly milk paint). I’m waiting for the counters to be installed to make a final decision, but want to have a good idea/direction in the meantime. Any & all ideas welcome! Thanks!