For KDE, click with the right button on the KDE Menu Icon and select "Modify applications".
Select the Filezilla icon in the menu editor.
Add: "GTK2_RC_FILES=/usr/share/themes/QtCurve/gtk-2.0/gtkrc " (with a space at the end) just before "filezilla" into the Command field to have: "GTK2_RC_FILES=/usr/share/themes/QtCurve/gtk-2.0/gtkrc filezilla"...
Apply.
Click on the Filezilla icon to start filezilla from the KDE menu launcher.
In this way you can keep default gtk theme for all other applications.
I suggest this workaround:
For KDE, click with the right button on the KDE Menu Icon and select "Modify applications".
Select the Filezilla icon in the menu editor.
Add: "GTK2_RC_ FILES=/ usr/share/ themes/ QtCurve/ gtk-2.0/ gtkrc " (with a space at the end) just before "filezilla" into the Command field to have: "GTK2_RC_ FILES=/ usr/share/ themes/ QtCurve/ gtk-2.0/ gtkrc filezilla"...
Apply.
Click on the Filezilla icon to start filezilla from the KDE menu launcher.
In this way you can keep default gtk theme for all other applications.