How to define a theme for specific application in kde plasma?

I use the following three variables:

XDG_CURRENT_DESKTOP=GNOME
QT_QPA_PLATFORMTHEME=lxqt
GTK_THEME=Default

Giving the advice of Felix, I tried the following with amazing results:

Packet Tracer without the fix: Here the text below the router is barely visible.

Packet Tracer with the fix: The text is now visible, and the general theme is consistent.

Here's what I did:

1. Locate the *.desktop file of the application. (In my case: /usr/local/share/applications/cisco-pt.desktop)
2. Make a copy of it and paste it in ~/.local/share/applications
3. In the line where is Exec=... add what Felix said. Just before the regular command for executing the application.

So the final desktop entry would be something like this:

[Desktop Entry]
Type=Application
Exec=XDG_CURRENT_DESKTOP=GNOME /opt/pt/packettracer %f
Name=Packet Tracer 8.0.0
Icon=/opt/pt/art/app.png
Terminal=false
StartupNotify=true
MimeType=application/x-pkt;application/x-pka;application/x-pkz;