How long are you staying there? What's your budget? What would your landlord allow you to do? In my first apartment, I simply cleaned everything really well (the landlord re-caulked), got a fun shower curtain and a cute little bath rug and considered it "done".

Staying here for years! My budget is lower preferably. Should I paint the cabinet above the toilet to match the dark sink? And what colour of curtain do you suggest? Thank you

There's no "should" in decorating a space. It's what you want/can do to it. I like my bathrooms bright and airy so I'd paint the walls white and get a fun, colourful shower curtain. Perhaps this jungle one from Urban Outfitters (loving bright white and green these days.) And I wouldn't paint the above sink cabinet to match the vanity. I doubt the paint would take well and I wouldn't want to make it darker. Perhaps I'd just put contact paper on the cabinet doors for colour. (Is there a good fan in this bathroom?)

Assuming you have a landlord, what are you allowed to change? Like suzmtl, I would paint all of the walls white (coordinating with the white shower surround) to keep things bright and airy if you are allowed.

I would definitely add a fun colorful shower curtain.

I would be a little worried that the cabinet above the toilet wouldn't take paint well. It's tough to tell from the picture. How about adding some potted moisture-loving low-light plants? You might be able to change out the knobs with something fun.