If one tries to theme icon in the activator of an eager plugin (e.g. org.mitk.gui.qt.matchpoint.core.helper), it does not work and the icon does not get themed.
Reason: Eager plugin activatores are activated before berryQtWorkbenchAdvisor::Initialize() is called. In this initialize() call the the style name is recived from the app preferences and SetStyle() of the style manager is called. At this point the style sheet is available. Before that the default style sheets have been added to the berryQtStyleManager, but none has been selected. So ThemeIcon() gets an empty style sheet and leaves the icon untouched.