RadDock's embedded base stylesheet - Dock.css contains dock's margin setting as well and it is loaded from the Telerik.Web.UI dll. If you add your custom skin reference between the head tags of the page dock's embedded base stylesheet will be loaded after your custom skin css file and it will override your setting. The easiest way to resolve the problem is to add the custom skin reference after the RadDock control (inside the body of the page). For your convenience I have attached a sample project which demonstrates how to achieve the explained above.
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