gtk: some Flatpak apps are styled incorrectly or become transparent

[brckd]

Reported in #693 (comment). The GTK styling for some Flatpak apps doesn't work and seems to cause issues such as transparency.

From my observations, apps that depend on org.gtk.Gtk3theme.adw-gtk3 are affected by this. Since Stylix also provides adw-gtk3, perhaps the conflicting definitions for it cause this issue?

        ID                                             Branch                 Op            Remote             Download
 1.     org.freedesktop.Platform.GL.default            23.08                  i             flathub            < 168,3 MB
 2.     org.freedesktop.Platform.GL.default            23.08-extra            i             flathub            < 168,3 MB
 3.     org.freedesktop.Platform.openh264              2.4.1                  i             flathub            < 976,5 kB
 4.     org.gnome.Platform.Locale                      46                     i             flathub            < 379,2 MB (partial)
 5.     org.gtk.Gtk3theme.adw-gtk3                     3.22                   i             flathub            < 153,2 kB
 6.     org.gnome.Platform                             46                     i             flathub            < 364,0 MB
 7.     com.github.flxzt.rnote                         stable                 i             flathub             < 20,5 MB

Proceed with these changes to the user installation? [Y/n]: 

Here are some of the errors raised when running such an affected application

(rnote:2): Gtk-WARNING **: 16:30:01.070: Theme parser error: gtk.css:7:38-39: Unexpected data at end of RGB() argument
(rnote:2): Gtk-WARNING **: 16:30:01.070: Theme parser error: gtk.css:10:28-34: Expected a valid color.
(rnote:2): Gtk-WARNING **: 16:30:01.070: Theme parser error: libadwaita.css:1:1-6: Unknown name of pseudo-class
(rnote:2): Gtk-WARNING **: 16:30:01.070: Theme parser error: libadwaita.css:54:22-26: Expected a valid color.
(rnote:2): Gtk-WARNING **: 16:30:01.070: Theme parser error: libadwaita.css:62:363-380: No property named "--accent-bg-color"

Uninstalling the org.gtk.Gtk3theme.adw-gtk3 dependency didn't change anything.

[brckd]

The current fix for this is to set stylix.targets.gtk.flatpakSupport.enable = false.