Would you consider changing the countertop as well? I think painting the vanity a light color: white, light blue, gray would help brighten the space. But I also think a new white countertop would really help a lot.

I would love to do that but I wasn't sure if it would create a whole mess exposing the tile it is grouted to. I suppose a prefab white cabinet with a carrara top is what you are thinking?

If I don't/can't do that and just paint the cabinet, I also thought white but then was concerned it might be too stark and out of place looking - too much of a pop - among all the dark. That's why I started going down the royal blue path.

Just to get an idea of a white vanity:

MOTINI Dimmable Bathroom Mirror With LED Light 36"x24" · More Info

Hammered Nickel Farley Oval Handcrafted Nickel Sink · More Info

Luxury Cotton Towel Set- 2 Piece Set Zero Twist Cotton, Navy/White · More Info

Or

6 Piece Complete Bathroom Towel Set- Navy Lavish Home · More Info

Or

Seaford Stripe 6 Piece Bath Towel Set in Blue · More Info

Window curtains

Starfish Fabric Sea Star Blue Barkcloth, Standard Cut · More Info

Bath mat

Pebble Look Bathtub Mat, Gray · More Info

Is the floor staying? Bringing in a light colored tile (or a patterned one with that blue in it) would help brighten things up. Barring that, make everything else white except towels. New towel bars or hooks can be nickel, as can the new sink faucet.

Yes - Trying not to replace anything big or attached - just looking to see what impact I can make with paint and decor. Thank you for your input! That makes sense. I was just concerned the white cabinet would pop in an unnatural way. What do you think about a mirror frame and lighting?

A neutral white wall color will look similar to the pale blue that is there and the shower will still be a strong feature in this room. I might paint the vanity white to match the trim and toilet and lighten it up, the square nobs repeat the small square tile shape so wouldn't necessarily need to change those. This wallpaper has a modern coastal feel and could balance the distribution of the blue tones throughout this space and you could also break it up with white wainscoting too. I like that this paper has the two tones of blue found in the majority of your shower tiles. Nextwall Paradise Palm Coastal Blue Botanical vinyl peel & stick sold through Home Depot and a grey version is available at Lowes. A rug with a grey and white stripe could add to the coastal feel as well.

It would be worth discussing replacement of the countertop with a pro who could tell you if it can be done without damaging the adjacent tile. It would certainly lighten the room. Take some close ups of the adjoining surfaces and show them to a tile store (not a big box) or a contractor. Or maybe if you post them here a professional might offer an opinion.

Some visual. I think white vanity, white walls with black countertop and black or nickel hardware would look good.

You can paint floor tile. Especially since it's such a small area, I'd try it.

What color runner? I think painting the floor would be problematic because it currently repeats the color of the shower floor.

And you don't think the white is too start sandwiched between the dark granite top and the dark gray floor? A good royal blue would be too much for the cabinet. Right?

The deep blue tiles of the shower are the stars in this room. You said at the beginning, this is not a whole redo, so I’d paint the vanity white; it looks great in Celery’s rendition. And then don’t worry about the floor; it relates well to the countertop. When, and if, you decide to change the floor, come back here for some more suggestions, but I think you’ll be pleased by making that one major change to the vanity.

By ”runner” do you mean bath mat? Just try to match the gray stone look of the floor.

If you click on the pic I posted of an LED mirror you will see one example of it being used in a bathroom where the vanity has a black countertop and a white base, but it is an off-white, barely veering into gray. Think “stoneware white”

For the floor you could use a luxury vinyl wood look plank that can be installed over the floor you have, no demo. Dal tile also has a porcelain click together plank that can also be installed over existing floor, called Revo tile. I agree also with painting or replacing vanity and top with a lighter color and let that brilliant blue tile be the star. Good luck