how it's going on?
I was upset with the fact that the icons of one of my most used applications, like Vivaldi and Spotify, didn't have the Elementary OS style. So I tried to use AppEditor from Flathub and it didn't work. Then I tried to use Menu Libre installed from Apt and it worked like a charm! I took icons from Elementary OS applications and used them on these Apps. But at the same time, I couldn't see Vivaldi and Spotify on the notification center anymore.
EDIT: there was an "Other" option in the notification center. Disabling the sound there worked for now.