There are actually many different ways you can customize the UI to your liking, including themes among other things. You can also run GIMP in window mode which encapsulates all sub windows into a single app window, just like the other apps you mentioned.
If none of these are your cup of tea, please make a PR - It’s open source. :)
If none of these are your cup of tea, please make a PR - It’s open source. :)
That said, Krita and Blender are very very nice.