The ceramic needs to be built out. In the following pic you'll see on either side of the window, where the accent stripe comes into the outlets and switches. That stripe, without doing anything to the tumbled marble, would've stood proud by about a shy 1/8", in which case, as you're finding out now, there'd be a gap behind the face plate. You might want to pull the ceramic pieces and build them out a bit.

i dont' think i would do white... very difficult to get right with the cabs...if u can find the right shade and u like it, go for it... maybe a very soft shade of blue to pull in your cabs... but of course it has to be in the same tone as the cabs... a matte tile or satin - light sheen. i would not do a gloss/glass tile i hesitate to say gray b/c its been done to death.... maybe a gray/blue? i do like a 6x6 tile... just to get away from a subway... whatever u choose, have grout to match as close as u can, so the b/s just sits in the background. bring home lots a samples to see in your space with your lights and natural light.

This is what we were hoping it would look like.

The top end choice would be to use a beautifully figured piece of alexandrite slab as the backsplash and focal point. It depends on your layout and sight lines. Whether the alexandrite is polished or honed and lighting come into play. Tile choices require taking your component samples to some tile stores and seeing what you like. Patterns like subway, herringbone, stacked, chevron, etc. depend on your interior decor style. Get samples to compare in your home lighting. And compare grout. Urethane and epoxy grout resist staining for lighter colors. Post pictures of the options for feedback.