This sample demonstrates the Tree Grid header template feature. In this sample, we have shown custom icons in the column headers.
The Tree Grid provides a way to define a custom element in header element. columns->headertemplate property accepts
either string or HTML element`s ID value, which will be used as the template for the header cell.
In this demo, we have render customized template for all column headers.
More information about Header template can be found in documentation section.