Apps running via Flatpak do not respect the desktop's theme settings.

Affected scenarios:

• running a GNOME Flatpak app on non-GNOME desktop
• running a KDE Flatpak app on GNOME (usually Qt / KDE apps get restyled to look like GTK+)
• distros that use custom desktops (Ubuntu, Elementary)
• theme and icon set changed in gnome-tweak-tool
• theme changed in KDE