Fix GUI issues with KDE Plasma dark theme

mkdir ~/.config-pt
cd ~/.config
cp -rf dconf gtk-3.0 gtk-4.0 xsettingsd ~/.config-pt

1. Right-click on Menu button.
2. Click Edit Applications.
3. Select Packet Tracer.
4. Add XDG_CONFIG_HOME=/home/jas/.config-pt to Environment variables.
5. Save.

Source. Thanks, u/AtomHeartSon!