I’d lean into white accessories to brighten the space. Even if you don’t plan to use the shower it might come in handy so I’d get a rod & hang a pretty textured white curtain. Do get a rug for the floor - the largest one width & length that fits space in front of vanity & toilet. Get a rug with a textured pattern & white background. If any color, used the tile colors. Place rolled up white towels & extra TP on shelf under sink. Hang a very large & lightweight wrapped canvas art on the wall that’s at right angles to the toilet. The lightweight canvas can be hung using 3M picture hanger strips since that looks like a tile wall. Maybe free standing TP holder as I don’t see one on wall by toilet. Use a 3M silver color hook as a hand towel holder by the sink.

You are renting, so I would not stick anything on tile that might cause issues when you move out. Better light bulbs and/or light fixture will probably help. A light fixture can easily be installed and removed in a rental and you can even take the better light fixture with you when you move out.

I would paint the vanity either charcoal or black as well as the window frame. Install a plain white film on the window giving it a frosted glass look. Replace the light fixture for a black one, replace the mirror for one with a black frame, replace the vanity and shower faucets for black. I would install a glass shower surround. Place wicker baskets on the vanity shelf and a natural fibre mat.