The graphic options (see "Setting graphic options in diagrams") defined for a given element in a diagram can be cloned and then applied to another element in the same diagram.
To clone an element’s graphic options, simply select the element and click on the 
icon in the diagram toolbar on the top right of the diagram, and then click on the element to which you want to apply the first element’s graphic options. The second element is then transformed using the graphic options cloned from the first selected element.