Modifying the css of a widget C++ GTK

I am trying to edit the css of a tree view model (list store), I have set everything up using this code and it works perfectly:

auto css_provider = Gtk::CssProvider::create();
css_provider->load_from_path("style.css");

Gtk::StyleContext::add_provider_for_screen
                    (Gdk::Screen::get_default(), css_provider, 
                    GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);

This code successfully loads in the css file and I can change certain properties of the treeview, ive tested padding and that works fine, it applies padding to each row in the liststore:

treeview.view {
    padding: 20px;
}

or

treeview * {
    margin: 10px;
}

But if I try to reference the name of the treeview, it doesn't work (no errors or warnings):

#m_TreeView {           /* or m_TreeView.view */
    padding: 20px;
}

My main goal is to apply a margin between each row, but this also doesn't seem to work (no errors or warnings), I want to apply a margin to each row but reference the specific name of the tree view as I would have multiple tree views in my application:

treeview.view {
    margin: 20px;
}

Tried to access the rows and tried to access the list stores children (*):

treeview row {
    margin: 10px;
}

+

liststore * {
    margin: 10px;
}