<%@ Page Language="C#" MasterPageFile="~/MyMasterPage.master" %>
<asp:content id="MyContent" contentplaceholderid="MyContentPH" 
             runat="server" > 
   <script language="C#" runat="server">
      void Page_Load(object sender, EventArgs e) {
         HtmlTableCell cell = 
              Master.FindControl("_CellHeader") as HtmlTableCell;
         cell.BgColor = "white";
      }
   </script>
   <asp:label id="lbl"          Height="100%"     Width="100%"
              BackColor="Black" ForeColor="White" runat="server" 
              Font-Size="XX-Large">Content of Default.aspx</asp:label>  
</asp:content>