I made my own GIMP themes and am happy with how it looks now. Dark and middle grey if anyone is interested. I have clients looking over my shoulder every week and no one has even asked what I'm using.

Been using both Photoshop and GIMP for professional reproduction work for years and both have their pros and cons in terms of their behaviour and layout.

Having taken some time to learn GIMP (which I also had to do once upon a time with Photoshop btw), I personally don't see it as a big priority for them to work on the UI/UX.

Yes a few things could benefit from a tweak, but I don't think it needs an overhaul.