Any control, even a specially styled div can be modified in the server-side by adding the runat="server"
attribute to it.
So in your aspx, you can do something like this:
<div class='message-dialog bg-color-blue fg-color-white' style='width: 50%; z-index:1000;' runat="server" id="myDiv" />
And in code behind do whatever you want... For instance:
myDiv.Controls.Add(new Label() { Text = "Hello World" });
or even
myDiv.InnerHtml = "<h1>Hello World</h1>";
Using Response.Write
is probably the wrong way to go in this situation...