private void dataGridView1_CellFormatting(object sender, DataGridViewCellFormattingEventArgs e)
{
  var dataGridView = sender as DataGridView;
  if (dataGridView.Rows[e.RowIndex].Selected)
  {
    e.CellStyle.Font = new Font(e.CellStyle.Font, FontStyle.Bold);
    // edit: to change the background color:
    e.CellStyle.SelectionBackColor = Color.Coral;
  }
}

private void dg_RowEnter(object sender, DataGridViewCellEventArgs e)
{
    System.Windows.Forms.DataGridViewCellStyle boldStyle = new System.Windows.Forms.DataGridViewCellStyle();
    boldStyle.Font = new System.Drawing.Font("Microsoft Sans Serif", 8.25F, System.Drawing.FontStyle.Bold);
    dg.Rows[e.RowIndex].DefaultCellStyle = boldStyle;
}

private void dg_RowLeave(object sender, DataGridViewCellEventArgs e)
{
    System.Windows.Forms.DataGridViewCellStyle norStyle = new System.Windows.Forms.DataGridViewCellStyle();
    norStyle.Font = new System.Drawing.Font("Microsoft Sans Serif", 8.25F, System.Drawing.FontStyle.Regular);
    dg.Rows[e.RowIndex].DefaultCellStyle = norStyle;
}

if (e.Row.RowType == DataControlRowType.DataRow)
{
 if (e.Row.Cells[rowIndex].Text == "Total")
 {
  e.Row.Font.Bold = true;
 }
}