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You'll need to do a string replace to fix the url. Replace `open.spotify.com` with `i.scdn.co`.
As far as setting the title bar to a dark theme variant goes, this is one of the bugs/papercuts that the unofficial flatpak works around.I actually wrote the script that fixes this issue in the flatpak wrapper. https://github.com/flathub/com.spotify.Client/blob/master/set-dark-theme-variant.py