If you wish to make your linphone-desktop experience more similar to
that of the AppImage (i.e. the old-timey GTK2 theme) then install the
package qt5-gtk2-platformtheme and start linphone like this:

  GDK_BACKEND=x11 QT_QPA_PLATFORMTHEME=gtk2 linphone

 -- Dennis Filder <d.filder@web.de>, Sat, 11 Dec 2021 19:54:28 +0100