If you like the darker wood tone of the shelf a wood look tile floor that was lighter but had dark tones in it could work. There is a the risk with the more multi-toned floors to look a bit rustic and that may not be what you are going for. Here is an example of a Home Depot tile Montagna Rustic Bay The picture above is a bathroom that I recently designed that has the wood look ceramic tile on the walls, a more rustic multi-color kind than the home depot link. I think the dark wood colors look nice with grey as well. Even if you are going with vinyl tile, there should be similar options in vinyl. Good luck!

Since you're looking for harmony through the whole apartment, can you pull a color for the bedroom's accent wall from the den's wallpaper or a color with a similar tone? What colors are in the wallpaper? Or since your accent colors in the navy living room are gold and white, maybe a bold gold for the accent wall? I don't think a teal would flow very well from the navy.

Anything you like. Without details or photos we are taking a stab in the dark. High-gloss oak can be any colour from off-white to black. Depending on stain, no stain, clear coat or oil based. The room's direction, your personal preference, your lighting, your furniture, etc all play a part. I'm a lover of Scandinavian Modern so I like white walls with plenty of colours in my soft furniture (like a purple painting + teal area rugs + high-gloss red cabinets). White is always a good choice (primer white is even better) until you figure out YOUR design preferences.

Thanks for the ideas! I'd like to leave the stones as is. If it makes a difference, I also ordered a medium gray couch and black TV stand.