title: How to set style on a specific header in ag-grid
body: <p>Im' trying to set style on a specific header in an ag-grid array generated dynamically. How Can I do this without manipulating the DOM ?
  This is a timeslot array and I want to highlight the current day.</p>
  
  <p>I already tried to apply cellStyle, but it can't affect headers. Do you have some ideas ?</p>
  
  <p>Some code generating the array :</p>
  
  <pre><code>this.tmpDefs = [{ headerName: '', field: 'user', width: 90, suppressSizeToFit: true }];
  
  for(wantedDays){
     this.tmpDefs.push([Some generated children])
  }
  
  this.gridApi.setColumnDefs(this.tmpDefs);
  </code></pre>