Export to excel header underline is not working

GridView1.HeaderRow.ForeColor = Drawing.Color.White
GridView1.HeaderRow.BackColor = Drawing.Color.FromArgb(75, 130, 201)
GridView1.HeaderStyle.Font.Underline = True

GridView1.HeaderRow.Style.Add("text-decoration", "Underline")
For Each cell As TableCell In GridView1.HeaderRow.Cells
cell.BackColor = GridView1.HeaderStyle.BackColor
cell.ForeColor = GridView1.HeaderStyle.ForeColor
GridView1.HeaderStyle.Font.Underline = True
GridView1.HeaderRow.Cells(0).Style.Add("text-decoration", "Underline")
Next

For Each row As GridViewRow In GridView1.Rows

For Each cell As TableCell In row.Cells
If row.RowIndex Mod 2 = 0 Then
row.BackColor = Drawing.Color.FromArgb(235, 235, 235)
cell.BackColor = GridView1.AlternatingRowStyle.BackColor
Else
row.BackColor = Drawing.Color.FromArgb(211, 211, 211)
cell.BackColor = GridView1.RowStyle.BackColor
End If
cell.CssClass = "textmode"
Next
Next
GridView1.RenderControl(hw)

Dim style As String = "<style> .textmode { } </style>"
Response.Write(style)
Response.Output.Write(sw.ToString())
Response.Flush()
Response.[End]()