<property
           name="applicationCSS"
           value="platform:/plugin/com.example.rcp4/css/default.css">
</property>

@import url("platform:/plugin/org.eclipse.ui.themes/css/e4_default_win7.css");

<property
       name="applicationcssResources"
       value="platform:/plugin/org.eclipse.ui.themes/images/">
 </property>