A server computing system stores a base definition file containing a hierarchy of elements defining a default control surface. In addition, the server computing system stores an extension definition file containing one or more elements defining a customized control surface based on the default control surface. The server computing system generates a modified version of the base definition file in which each element in the base definition file having an identifier corresponding to an identifier of an element in the extension definition file is modified in a way indicated by the element in the extension definition file. The server computing system uses the modified version of the base definition file to generate a resource used by a client computing system to present the customized control surface in a web page.