Style extensibility is provided to shapes in a diagram by editing text files. A structured data model is used to render the diagram representing items provided by the user in a text-based format such as a bulleted list. A style engine determines style properties for style labels assigned to key shapes of the diagram. Style properties include features associated with the shapes and their content that preserve an overall look-and-feel of the diagram. Color scheme(s) are also provided through the style labels. A layout engine uses layout definitions based on user input and default parameters along with style label assignments to render a presentation model. Style properties may be modified and new styles added after the diagram is rendered.