I tested this
Yes, ok, everything works fine to me as well. But as soon as you change the code and add to the dockcommand tag the property CssClass="MyCustomStyle" here comes the problem: the command appears in the titlebar and works properly but the Icon, defined in MyCustomStyle is not being displayed. In place of the right Icon, your default Collapse Icon has showned.
Following the very simple code on my Css:
Looking forward to hearing from you.