Acumatica Styling a specific column/cell in PXView?

Question

I am VERY new to Acumatica in coding. I am trying to change the color of a cell on the header page of AR303000. (see code below). Obviously this is not working. I receive the error message of PXFormViewEventArgs does not exist in the namespace PX.Web.UI.
Can anyone direct me as to what namespace I should be using and correct my code to make this work? Thank you,

using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using PX.Common;
using PX.Data;
using PX.SM;
using PX.Objects.AR.CCPaymentProcessing;
using PX.Objects.AR.Repositories;
using PX.Objects.Common;
using PX.Objects.Common.Discount;
using PX.Objects.CA;
using PX.Objects.CM;
using PX.Objects.CR;
using PX.Objects.CR.Extensions;
using PX.Objects.CS;
using PX.Objects.SO;
using PX.Objects.AR.CCPaymentProcessing.Helpers;
using PX.Data.BQL.Fluent;
using PX.Data.BQL;
using PX.Data.Descriptor;
using CashAccountAttribute = PX.Objects.GL.CashAccountAttribute;
using PX.Objects.GL.Helpers;
using PX.Objects.TX;
using PX.Objects.IN;
using PX.Objects.CR.Extensions.Relational;
using PX.Objects.CR.Extensions.CRCreateActions;
using PX.Objects.GDPR;
using PX.Objects.GraphExtensions.ExtendBAccount;
using PX.Data.ReferentialIntegrity.Attributes;
using CRLocation = PX.Objects.CR.Standalone.Location;
using PX.Objects;
using PX.Objects.AR;

namespace PX.Objects.AR
{
  public class CustomerMaint_Extension : PXGraphExtension<CustomerMaint>
  {
  

    protected void Page_Load(object sender, EventArgs e)
    {
      Style escalated = new Style();
      escalated.ForeColor = System.Drawing.Color.Red;
      this.Page.Header.StyleSheet.CreateStyleRule(escalated, this, ".CssEscalated");

      Style rowStyle = new Style();
      rowStyle.BackColor = System.Drawing.Color.Red;
      this.Page.Header.StyleSheet.CreateStyleRule(rowStyle, this, ".CssRowStyle");

      Style cellStyle = new Style();
      cellStyle.BackColor = System.Drawing.Color.Aqua;
      this.Page.Header.StyleSheet.CreateStyleRule(cellStyle, this, ".CssCellStyle");

      Style highlightStyle = new Style();
      highlightStyle.BackColor = System.Drawing.Color.Yellow;
      this.Page.Header.StyleSheet.CreateStyleRule(highlightStyle, this, ".CssHighlightStyle");
  
    }
    
    protected void Customer_RowSelected(PXCache sender, PXRowSelectedEventArgs e, PX.Web.UI.PXFormViewEventArgs f)
      {
        var customer = (CR.BAccount)e.Row;
        var customerExt = customer.GetExtension<CR.BAccountExt>();
        if (customerExt != null)
          {
            /*I want to change the color of this cell in the form*/
            customerExt.UsrReadOnlyAcctName = customer.AcctName;
          }
      }
  
    #region Event Handlers

    #endregion
  }
}```