Blank menus, QGtkStyle could not resolve GTK

[probonopd]

$ /home/me/Downloads/scribus-1.5.4-linux-x86_64.AppImage 
QGtkStyle could not resolve GTK. Make sure you have installed the proper libraries.

$ cat /etc/os-release 
NAME="Ubuntu"
VERSION="18.04 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic

[undertherain]

for a completely different software, which was in addition running as appimage, I solved very similar error message with QT_STYLE_OVERRIDE=gtk2

but knowing which exactly libraries are supposedly missing would be nice of course....

[probonopd]

Maybe the easiest and most robust solution would be to just remove Gtk theming support altogether by not shipping those Qt plugins. Things will not look native on Gtk systems then, but will at least not break.