Provide dark-mode toggle for flatpak
Feature
With the move to Libawaita, the whole way of handling light and dark themes is changes. This creates a unique problem for flatpaks, since flatpaks might be run on systems who don't actually support the new light-or-dark mode settings.
For that reason, it would be beneficial to introduce the toggle for light-or-dark mode. It should initially be whatever the environment prefers, but in case the environment doesn't have such a settings, the user should be able to manually define this.
Useful toggle in Easy Effects for everybody not running Arch of Fedora 36
Background
I reported a similar issue over at Lollypop: World/lollypop#2905 (closed)
Edited by Kevin Degeling