From your questions it sounds a bit like you are asking "How can I use CSS to customize a single instance of a control that use standard styles?"
Let's say you have a style that is normally applied to all your text boxes:
<asp:TextBox ID="TextBox2" runat="server" CssClass="myTextBox"></asp:TextBox>
But you want to modify this one a bit. Maybe you want a green border instead of the standard one. So you create a style for the green border. You can supplement the standard style by adding another style class to the control:
<asp:TextBox ID="TextBox2" runat="server" CssClass="myTextBox greenBorder"></asp:TextBox>
As long as the classes are assigned to the control in the correct order and the style rules are in the correct order to cascade properly, you can chain the css classes on a single control. (It might even be the case that the style application will be dictated by the classes on the control rather than the normal cascade rules.)