Edit imported css file with css designer

So I created custom.css file containing a few classes and attached it to my pages inside wappler. Although I can find the classes when I search in the CSS design panel, I am unable to get new rules done via css designer to update the file. Is this how it is supposed to be or I am missing something.

lets say i added the code below outside Wappler and I wanted to use the CSS designer to add additional rules like border-bottom: Is wappler able to update this file? It is not happening for me hence my asking.

  background-color: #3c3c3c;
  padding-top: 20px;
  padding-bottom: 20px;

What do you mean you are unable to do this?