How can I change font css for a specific Task Column in Gantt Chart

Each row header column cell is marked with several CSS classes. One of them is column-specific and includes the column number.

Example: Column #2 (indexed from 0) with the default theme ("gantt_default") uses the following CSS class:

"gantt_default_rowheadercol1"

In addition to that, you can use BeforeTaskRender event handler to specify custom CSS class that will be applied to all row header columns of a given row:

protected override void OnBeforeTaskRender(BeforeTaskRenderArgs e)
{
    if (e.Row.Columns.Count == 3)
    {
        e.Row.CssClass = "my-row";
    }
}

See also:
https://doc.daypilot.org/gantt/row-customization/

Then you can simple add a custom CSS which targets the cell using a combination of those classes:

.gantt_default_rowheadercol1.my-row .gantt_default_rowheader_inner {
  color: #ccc:
  background-color: #fff;
}