Add a Padding Color with a dark color as one of the options. Or add as a default in GIMP when installing.
There is a reason why Photoshop has a dark Padding Color by default when installing. This creates a contrast between the standard white canvas and between the bars and tools, in addition, it helps to protect the user's vision. Finally, it ends up making the program more beautiful precisely because of the contrast that is created.
Now look at the default color when installing GIMP, or when using the dark theme:
There is no contrast between the bars and the padding color.
So what could you do to solve this problem:
Add a Padding Color option with a dark gray by default among those available (something close to # 1a1818):
I also recommend that you add this color as the initial color when installing the program