why eclipse-rcp theme color override not work?

I am using helios.

here is my plugin.xml:

<extension
         point="org.eclipse.ui.themes">
      <theme
            id="de.spiritlink.custom.ui.theme"
            name="Custom Theme">
         <colorOverride
               id="org.eclipse.ui.workbench.ACTIVE_TAB_BG_END"
               value="255,0,0">
         </colorOverride>
         <colorOverride
               id="org.eclipse.ui.workbench.ACTIVE_TAB_BG_START"
               value="0,255,0">
         </c开发者_开发技巧olorOverride>
      </theme>
   </extension>

Are you sure you have set theme to you defined in Runtime?