Usually land lords are appreciative of tennants painting the walls simply because it means for a fresh coat of paint that they wouldn't have to do other wise.

Paint and everything involved in painting is rather cheap, just will take a day or maybe two at the most of your life would probably be the biggest investment.

Ask your landlord if you can have permission to repaint the walls, first ask if he or she is even willing to pay for the paint and supplies if you are willing to do the labor yourself (50% chance he or she will agree) and 90% chance he or she will agree for you to do it with you paying for the supplies.

I just think a new paint job would be the biggest contributing factor for a new look and vibe to your room as well as being the cheapest option at the same time.

Completely up to you though

EDIT: These are the Curtains I ordered that look exactly like yours, just longer

So if you want the same ones just longer then maybe can get them here. They might be too long though, but there are options on that same page for shorter, idk if that's too short for your window/room though.