Warning about theme integration for GTK2, GTK3, Qt4 and Qt5:

GTK2 and GTK3 theme will be overidden by desktop session manager no
matter what .gtkrc-2.0 and/or .config/gtk-3.0/settings.ini are
configured properly.

Every "desktop environment" has it's own application to change this.

Under LXDE run lxappearance to set NsCDE theme.
Under MATE, run mate-appearance-properties.
Under KDE, run settings, or "kcmshell5 kde-gtk-config"
Under GNOME, gnome-tweaks (make sure gtk-3.0 symlink in points to 

You will also have to set "sh $HOME/.NsCDE/Xset.conf". Putting it
in $HOME/.config/autostart as "xset.desktop" manually or by desk
tool does not give expected result. You should probably use
$FVWM_USERDIR/Init.fvwmconf SessionInitFunction and make it Exec.

Also, from autostarted applications which are outside FVWM process
group, NSCDE_* and FVWM_* and any other variables set from NsCDE
configurations will not be set. XAPPLRESDIR will be absent too.
FVWM's SessionInitFunction is your best chance.
