If you put the control and the expander in the table, you can set the background color in the tag as follows:
<td style="background-color: white;"> <asp:TextBox ID="myTextBox" MaxLength="1000" Width="250px" Height="250px" TextMode="MultiLine" Rows="10" Wrap="true" runat="server" /> <ajaxToolkit:HtmlEditorExtender ID="HtmlEditorExtender1" TargetControlID="myTextBox" runat="server"/> </td>
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