Stop removing link to external css

If I add a link to an external stylesheet to a topic, and save the topic, the link works fine. However, the next time I edit the topic, RoboHelp 2015 removes the link to the external stylesheet. I am required to use this external stylesheet for this type of topic because the stylesheet is used by a number of other authors and it refers to externally hosted image files. How do I either get RoboHelp to stop removing the link to the external stylesheet or to allow me to reference an external stylesheet in the topic properties?