Feature suggestion: support freedesktop dark style preference

[idoric]

SongRec doesn't follow the freedesktop dark style preference, neither during operation nor at startup. Concretely, to take my example, under GNOME, when my system switches to dark style automatically for the night, SongRec remains in light style.

GTK3 + libhandy, GTK4 + libadwaita or a recent version of Qt manage directly the freedesktop dark style preference, but it's possible to manually access the settings portal and fetch the preference via DBus: https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.Settings.html.