JW Player how to use your skin?

Question

css

Good evening!
Help to deal with JW Player. I need to style it a bit for my purposes.
On the Internet, I dug up not so much (as I understand it, the last, 7th version) information.
The code looks like this:

$(document).ready(function(){        
        jwplayer('mediaplayer').setup({
            'playlist': [{
              'sources': [
                {
                  'file': 'it.mp4'
                }
              ]
            }],
            'width': 480,
            'height': 320,
            skin: "myskinfile.css"
        });
    });

As I understood in skin: "myskinfile.css" my styles should be:

button{
  background-color: red;
  padding: 50px;
}

None of the elements are responsive. If you insert styles into html - the same way.

#body .jw_modified .jw-skin-seven .jw-display-icon-container  {
width: 110px;
border-radius: 10%;
border-width: 2px;
border-color: blue;
}

Tell me how can I change the style of the player?