Hang the shower curtain much higher and keep it closed. That will completely block the shower from view and you won't need to retile now. Save that for down the road when you don't have so much on your plate. (Sounds like you recently purchased an old home that needs a lot of other work.)

I like a dark masculine bathroom. I'd leave the vanity as is and paint the walls and ceiling a dark color - a deep moody grey? It would have to be right color to no look like a near miss or clash with the floor and counter.

New lighting. New mirror.

Or, you can paint the tiles white. Then you need to figure out the walls. Is that wallpaper?

Maybe replace it with a blue and white print wall paper.You can then have a white shower curtain but one a bit fancier.

Remove the wallpaper.
Replace the mirror.
And close the shower curtain.

" If I kept the floor and countertop...........? " You would be ignoring the best part of the room--original tiles in the tub surround.

Maybe so, but it looks like the previous owners ignored them too when they picked the rest of the finishes ;)

I would focus on removing the wallpaper, skinming/repairing the walls and getting a good paint color, that will be enough of a job!