Add a dark mode toggle in the appearance settings

[Asghwor]

Currently, to enable dark mode for Flatpak apps that either don't use GTK or don't use the Mint-Y theme for some reason (e.g. Flatseal) we need to run the following terminal command:

gsettings set org.gnome.desktop.interface color-scheme prefer-dark

It would be nice to have this toggle in the settings, perhaps changed automatically when the user selects a dark or light Mint-Y theme.