I too am downgrading to 54 until one of two things happen. Opera wises up and allows us to choose the window manger theme (ie give us the option to have Opera look like all other windows, especially in Linux + KDE) or I make the time to migrate all my bookmarks to Firefox.

I have used Opera for the past decade and I really love it, but Opera is now utterly hideous on Linux. Thank the heavens macOS forces Opera to obey its theming or I would migrating my bookmarks instead of posting this message.