How to change the style of Published ASPX pages

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ASP.NET

CSS

Hi,

I have an ASCX Web Control (Cartviper module for DNN). I want to customize the style of the control, without touching the Code. But, I think its a Published one. and I have the DLLS.
When I open the File AjaxAddtoCart.ascx in Visual Studio, Instead of Design, It shows an error. "Error Creating Control - cvNotificationUnknown server tag 'cv:CvNotificationControl'."
How can I reference this to the DLL ??
Tag Prefix & Control Tag is like this...

<%@ Register TagPrefix="cv" Namespace="CartViper.Modules.Store.WebControls" Assembly="CartViper.Modules.Store" %>
 <cv:CvNotificationControl  runat="server" ID="cvNotification" />