The icon theme selection menu does not show up icon themes that are Symlinked into the users default paths. (e.g. ~/.icons/MyTheme or in ~/.local/share/icons/MyTheme) In comparison: The lxde gtk-config gui "lxappearance" does show them properly and let me set 'em.
I found this is due to the check performed here: https://cgit.kde.org/kde-gtk-config.git/tree/src/iconthemesmodel.cpp#n50
Icons are now automatically synced from the Icons KCM, this this problem no longer occurs.