A comparison between Edge and Edge Chromium in start menu
In the picture above, Edge follows the theme settings of the system, while Chromium edge uses black for default. (I remember Google Chrome used approximately the same background color scheme). But the problem is, since most application or UWP uses system theme color as background color, you just don't want one black in all blue (system color) boxes.
Sometimes you just want the background to be consistent.
I know this problem could be fixed by editing msedge.VisualElementsManifest.xml by removing the color settings. But in that case, I have to do that every time there is an update. That can be really annoying.