Support dark mode preference

GNOME 42 will have a dark style preference, it would be good to support it.

• Follow the preference if applicable
• Adjust in-app dark style preferences, if there are any
• Ensure the app is usable and has decent contrast with dark appearance

Read more about this at the initiative, @exalm, @BrainBlasted and @msandova will help with the implementation of it. Let us know if you have any questions & thoughts.

From a quick review (on latest Flathub release) it seems that it only has to take care of #28 (closed) and opt-in to the preference in case the GTK 4 port does not land in time for 42.

Related: !19 (merged)