Its a control implemented in the Forums at Forums/Controls/ForumDisplaySettings.cs
you would need to change the namespace in your version and also change the control declaration to your own namespace everywhere it is referenced. You'll need to look in all the .ascx and .aspx files for it. The one in the ForumModule.ascx has the tag prefix declared at the top like this:
<%@ Register Namespace="mojoPortal.Web.ForumUI" Assembly="mojoPortal.Features.UI" TagPrefix="forum" %>
then the control is declared like this:
<forum:ForumDisplaySettings ID="displaySettings" runat="server" />
the main purpose of this control is to provide a set of properties that can be configured from the theme.skin file to control various aspects of the rendering, ie it was implemented as part of mobile support so that the forums could render differently in mobile devices as specified in the theme.skin file of a mobile skin. The ForumDisplaySettings control itlelf does not render anything but other controls check its properties for configurations that control their rendering.
You'll need to look around and everywhere it is used you need to declare the tag prefix with your namespace and assembly name. You should also use a different tag prefix otherwise it may clash with existing declarations for the forums.
Hope that helps,