public class MyRenderer : SpreadStreamExportRenderer
  {
      public override void SetCellValue(DataType dataType, object value)
      {
          DateTime date;
          if (value != null && DateTime.TryParse(value.ToString(), out date))
          {
              base.SetCellValue(DataType.DateTime, date);
              var cell = this.GetCell() as ICellExporter;
              cell.SetFormat(new SpreadCellFormat() { NumberFormat = "yyyy-MM-dd HH:mm:ss" });
              return;
          }
          base.SetCellValue(dataType, value);
      }
  }

 spreadStreamExport.RunExportAsync(FILENAME, new MyRenderer(), options);

 var cells = radDocument.EnumerateChildrenOfType<Telerik.Windows.Documents.Model.TableCell>();
 foreach (var cell in cells)
 {
     cell.Padding = new Padding(10);
 }

 StyleManager.ApplicationTheme = new Office2016Theme();