Color Scheme Unavailable on Flatpak/Not respecting GNOME light/dark mode

[tessssaract]

Description
There is no option to switch color scheme, and Pinta does not respect the dark/light theme toggle in GNOME.

To Reproduce

• Install Pinta via a flatpak (Flathub)
• Check the view tab
• Toggle light/dark mode in GNOME settings
• Observe Pinta remains in light mode

Additional Info

• I'm not sure if this is relevant at all, but I use an NVidia GPU with proprietary drivers.

Version
OS: Fedora 40 Silverblue
Version: 2.1.2 (via Flathub)

[cameronwhite]

I'm not entirely sure if there are any flatpak issues at play here, but it's possible that the GNOME light/dark toggle won't affect Pinta 2.1 since it's using GTK3. Pinta should be following the system GTK3 theme, but some themes which have a dark variant only use the dark mode when this is turned on by a setting (https://wiki.archlinux.org/title/GTK#Dark_theme_variant) or when an application specifically requests it, which Pinta 2.1 isn't doing.

This is expected to work in Pinta 2.2 which uses GTK4 and libadwaita, which automatically switches between the libadwaita light / dark modes based on the system preference.

Once flatpak builds are ready for Pinta 2.2 this should be tested to verify it does work under flatpak