How to fix incomplete CSS styling support for python gtk3 application in Debian 9.5 i386?

Question

linux

In Linux Mint 18.3 (64-bit), all of the styles below work as they should.

#lock_screen {
    background-image: url("background.jpg");
    background-position: center center;
    background-size: cover;
}

#lock_screen > GtkBox {
    background-color: rgba(0, 0, 0, 0.75);
    border-radius: 8px;
    font-size: 16px;
    font-family: "Noto Sans";
    border: 2px rgba(255, 255, 255, 1) solid;
    box-shadow: 0px 0px 4px rgba(0, 0, 0, 0.75);
}

#lock_screen GtkLabel {
    color: #fff;
}

#tonb_label {
    padding: 16px 0px 8px 0px;
    border-bottom: 2px rgba(255, 255, 255, 1) solid;
}

#status_icon {
    padding: 16px 8px 0px 8px;
}

#status_label {
    padding: 8px 8px 16px 8px;
}

In Debian 9.5 (32-bit), nested CSS selectors and all properties are ignored. The environment used is LXDE, but I tried to run it without the environment - the effect is the same. The following packages have been installed for the python Gtk3 application to work:
#lock_screen > GtkBox {...}
#lock_screen GtkLabel {...}

python3-gi python3-click python3-gi-cairo python3-cairo gir1.2-gtk-3.0 libglib2.0-dev libcairo2-dev libgirepository1.0-dev

UPDATE
When using only unique identifiers, everything works as it should.
However, that doesn't change the question why nested selectors work in Linux Mint 18.3 but not in Debian 9.5.