@j0achim The issue is google!
They added "dark mode support" for google search results and some other their services.
I had no issue with "normal opera and firefox" but now, they also show that white shadow around sign in button.
I tried something to solve it but I have to say, they didnt work well.
I also installed "google chrome (which i hate)" but result is same. In this case, only solution is a little quick script change. Or you can stop using this extension or simply disable it and give "google dark mode support" a chance. Looks satisfying!