Enable dark theme in Linux

[JasonLG1979]

You'll need to do a string replace to fix the url. Replace `open.spotify.com` with `i.scdn.co`.

[francie00]

Thanks for the suggestion, I could use it in one of my scripts!

[JasonLG1979]

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