I have a custom webpart called `Available` seminars:

[![webpart][1]][1]

I want to modify the width of certain columns.

On the Internet it is written that I have to use

    <style type='text/css'>  
        .ms-vh-div[DisplayName='<ColumnName>'] {  
            width: 250px;  
        }  
    </style> 

But the problem is that I can't edit the source of Available Seminars. (See the blue border in the image). I can edit the source of my content editor, but not of my custom webpart.

When I click on Edit Source, this shows up:

    <div class="ms-rtestate-read ms-rte-wpbox" contenteditable="false" unselectable="on">
       <div class="ms-rtestate-notify  ms-rtestate-read 75788cb9-fede-4c30-931f-34dfb6a75048" id="div_75788cb9-fede-4c30-931f-34dfb6a75048" unselectable="on">
       </div>
       <div id="vid_75788cb9-fede-4c30-931f-34dfb6a75048" unselectable="on" style="display: none;">
       </div>
    </div>
    <div class="ms-rtestate-read ms-rte-wpbox" contenteditable="false" unselectable="on">
       <div class="ms-rtestate-notify  ms-rtestate-read 1ba441c2-b2e9-42a7-b0cc-506e02119f5c" id="div_1ba441c2-b2e9-42a7-b0cc-506e02119f5c" unselectable="on">
       </div>
       <div id="vid_1ba441c2-b2e9-42a7-b0cc-506e02119f5c" unselectable="on" style="display: none;">
       </div>
    </div>
    <div class="ms-rtestate-read ms-rte-wpbox" contenteditable="false" unselectable="on">
       <div class="ms-rtestate-notify  ms-rtestate-read 92217f41-4296-4700-9b00-0a556b46141d" id="div_92217f41-4296-4700-9b00-0a556b46141d" unselectable="on">
       </div>
       <div id="vid_92217f41-4296-4700-9b00-0a556b46141d" unselectable="on" style="display: none;">
       </div>
    </div> 
    <br/>

And when I add 

    <style type='text/css'>  
        .ms-vh-div[DisplayName='<Title>'] {  
            width: 450px;  
        }  
    </style> 

then nothing happens.

Does someone know a solution to this?

Here's what shows up when I go to title with the dev tools:

[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/ntFIU.png
  [2]: https://i.sstatic.net/AooGo.png