I am exporting my data inside my Gridview control to Excel, however when I open the exported data then the font colours are not consistent, How is it possible to export the data with all text in black

This is my current code: 

        Response.Clear()
        Response.Buffer = True
        Response.AddHeader("content-disposition", "attachment;filename=GridViewExport.xls")
        Response.Charset = ""
        Response.ContentType = "application/vnd.ms-excel"
        Using sw As New StringWriter()
            Dim hw As New HtmlTextWriter(sw)

            'To Export all pages
            GridView1.AllowPaging = False
            Me.LoadDataGrid()

            GridView1.HeaderRow.BackColor = Drawing.Color.White
            For Each cell As TableCell In GridView1.HeaderRow.Cells
                cell.BackColor = GridView1.HeaderStyle.BackColor
            Next
            For Each row As GridViewRow In GridView1.Rows
                row.BackColor = Drawing.Color.White
                For Each cell As TableCell In row.Cells
                    If row.RowIndex Mod 2 = 0 Then
                        cell.BackColor = GridView1.AlternatingRowStyle.BackColor
                    Else
                        cell.BackColor = GridView1.RowStyle.BackColor
                    End If
                    cell.CssClass = "textmode"
                Next
            Next

            GridView1.RenderControl(hw)
            'style to format numbers to string
            Dim style As String = "<style> .textmode { } </style>"
            Response.Write(style)
            Response.Output.Write(sw.ToString())
            Response.Flush()
            Response.[End]()
        End Using