I found out that when I get a notification the Settings menu of that Notification (where you can say to not get notifications from that website) that the Text is white. So it´s hard to read the text since the Notification itself is white / grey-ish.
I am using MS Edge Dev Version 184.108.40.206 (Offizielles Build) dev(64-Bit) on Windows 7 SP1 (64-Bit).
@HotCakeX The problem is the notification that appears when I start Edge (mostly Reddit notifications). Their text is white, even if I use the dark theme. Also, the "Fertig" button means done in german.