A black roller blind or roman blind would work here ( the dark colour on your towels)

Can you sew? If not, you could use the iron on no sew tape and very easily make a pair of curtains, designed and made exactly to your measurements. Check the JoAnn's website for the product. Good luck.

Judging from the glass on the window, I gather privacy is not an issue.
Pick a fun pattern and just do a valance in a fun pattern with some colors you like. Adjust your towels, area rugs and art to go with accordingly.

I would not use curtains on this window as it's so close to the shower. Instead I would consider one of these solutions:
1. Shutters. The window is small and the shutters wouldnn't cost so very much and would be my absolute first choice for your window.

2. A bamboo shade mounted at ceiling height to give the window a bigger look.

3. Wood blinds, mounted within the window opening. Simple and effective.
Carolyn Albert-Kincl, ASID

I believe I would just put in plantation shutters, simple and understated elegance......just like the kitty!!! Love him!!!