lxappearance.desktop contains this line:
NotShowIn=GNOME;XFCE;
"git blame" shows that line was added for bug #3074182 to make sure lxappearance is not shown in GNOME and XFCE .
So please also add KDE into "NotShowIn", because KDE has its own module for controlling the appearance of GTK applications.
Maybe the better fix is use "OnlyShowIn=LXDE;" instead.
As mentioned in 3537359 I agree we should better use NotShowIn rather than OnlyShowIn.
Fixed in http://lxde.git.sourceforge.net/git/gitweb.cgi?p=lxde/lxappearance;a=commit;h=6235bf21