Select light/dark theme variant depending on Riot's theme setting

[aperezdc]

Riot has not a dark theme variant (element-hq/element-web#1635). It would be desirable that the GTK+ theme variant being used follows the user setting in Riot. This will be a little bit involved, as we would like to pick the user setting from Riot's preferences, which means:

• Adding a WebKitWebExtension to gain access to the DOM, or using an injected JS snippet plus user script messages (see WebKitUserContentManager).
• Listen for onclick events on the radio buttons used to select the theme.
  • On logout, we probably want to revert to the default (light) theme.
  • When Riot finishes loading, we check what is the selected variant, and apply it to the GTK+ theme variant selection if needed.
  • Also, remember the choice as a GSetting so Revolt does not initially start with a different theme variant than that used by Riot.
• Once this is done, it could be that running inside the Flatpak sandbox still triggers issue Dark GTK+ theme is not used when running in the Flatpak sandbox #11. Double check that.

[MightyCreak]

Dark theme in Riot has landed, element-hq/element-web#1635 is closed now.