Can I make GtkPlug transparent?

["李浩" <lihao kylinos cn>]

Hi, I encountered a problem when using GtkPlug and GtkSocket.


I use CSS to set the background of GtkSocket to red and then use CSS to make GtkPlug transparent. I think I 
can see the red color because GtkPlug has been set to transparent, but it's actually black. I can't figure 
out why the effect is not correct. Could someone help me out? My Gtk version is Gtk 3.18.


The CSS files are as follows.


/* socket.css */
GtkSocket {
        background-color:rgba(233,23,34,1);
}


/* plug.css */
GtkPlug {
        background-color: transparent;
}



The code is here:
[socket.c]  https://pastebin.com/68XLqFUF
[plug.c]  https://pastebin.com/Cp3BPHFh