Dark GTK+ theme on Microsoft Edge Dev 91.0.831.1 for Linux

I encourage you to file a detailed Feedback report so that the Edge-Linux team can understand, evaluate and correct, as needed.

But I am not sure that I understand what is wrong.

I checked to see how your screenshot compared to my results under different conditions, and I found that the Edge UI is consistent (and seemingly identical to your screenshot in all material respects) across Windows 10, Solus Budgie, Solus Plasma, Zorin 16 Beta, and Ubuntu 20.10, all set to dark or system default mode/theme.

In each case, the content is displayed against a white background, bars and menus are displayed against a dark background, and various icons are displayed either as white or grey. It looks to me as if the Edge defaults are standard across platforms. I suspect that this is intended.

I checked Firefox, too, in all the platforms, with identical results, just to see if Firefox behaved differently. It seems Firefox also defaults to a standard UI across platforms.

Within the confines/limitations of each platform (some offering more options than others) the Edge-Linux UI appearance can be changed by adopting different themes under Settings>Appearance, but I have not explored that option.  I use default settings.