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

Riot has not a dark theme variant (vector-im/riot-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 #11. Double check that.

aperezdc / revolt

Select light/dark theme variant depending on Riot's theme setting #38