This is the CssClass I apply to all of my WebTextEdit controls:

.txtBoxRegular {
    border-left: 1px solid #415f90;
    border-right: 1px solid #415f90;
    border-top: 1px solid #415f90;
    border-bottom: 1px solid #415f90;
}

As soon as I place a WebDialogWindow inside my master page, the above CssClass is ignored for all WebTextEdit controls inside my content pages - the borders do not show up. 

Does anyone have any idea why this would happen?

I've tried moving the WebDialogWindow to the top of the form and to the bottom but it didn't help.  I only get the borders back when I remove the WebDialogWindow from the master page completely.

There's nothing special about this WebDialogWindow - I copy it below:

    <igl:WebDialogWindow runat="server" ID="wdwDeletePrompt" InitialLocation="Centered"
        Height="150px" Width="286px" Modal="true" WindowState="Hidden"
      Style="line-height: normal;">
        <Header CaptionAlignment="Left" CaptionText="Delete">
            <CloseBox Visible="False" />
        </Header>
        <ContentPane CssClass="red" Font-Bold="true" BackColor="#FFFFFF">
            <Template>
                <div style="text-align: center; position: relative;">
                    <br /><asp:Label ID="lblPromptText" runat="Server" Text="Are you sure?"></asp:Label> <br />
                    <br />
                    <div style="position: absolute; right: 77px; top: 75px; height: 23px">
                        <img onclick="DeleteConfirm()" style="cursor: hand;" src="images/webdialogwindow/YesBtn.jpg"
                            alt="Yes"/>
                        <img onclick="DeleteCancel()" style="cursor: hand;" src="images/webdialogwindow/NoBtn.jpg"
                            alt="No"/>
                    </div>
                </div>
            </Template>
        </ContentPane>
    </igl:WebDialogWindow>