artwork/usr/share/themes/Lubuntu-default/gtk-3.0/gtk-widgets-assets-radiance.css

.menu .check {
    background-image: url("assets/check-menuitem-unselected.png");
}

.menu .radio {
    background-image: url("assets/check-menuitem-unselected.png");
}

.menu .check:active {
    background-image: url("assets/check-menuitem-dark.png");
}

.menu .check:active:insensitive {
    background-image: url("assets/check-menuitem-insensitive-dark.png");
}

.menu .radio:active {
    background-image: url("assets/radio-menuitem-dark.png");
}

.menu .radio:active:insensitive {
    background-image: url("assets/radio-menuitem-insensitive-dark.png");
}