How to change the color of one or more lines in the gridviewer?

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C#

ASP.NET

SQL

I'm developing a form, where name, task and date are added, forming a table (GridView).
Now I need to put a warning on the screen when the "date" column is expiring. I used Google a lot, I understood how the code works, but it's not running, it doesn't appear on the screen. The "nothing" message also doesn't display. I use C#, ASP.NET and SQL Server, in the latest versions (2020).

What I have tried:

protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
        {
            if (e.Row.RowType == DataControlRowType.DataRow)
            {
                DateTime dt = DateTime.ParseExact(e.Row.Cells[4].Text, "dd/MM/yyyy", System.Globalization.CultureInfo.InvariantCulture);
                if (dt < DateTime.Now)
                    e.Row.Style.Add("background-color", "grey");
            }
            else
            {
                Response.Write("nothing");
            }
        }