user/test/com/google/gwt/uibinder/test/client/gss/WidgetUsingGss.ui.xml

<ui:UiBinder xmlns:ui='urn:ui:com.google.gwt.uibinder'
             xmlns:g='urn:import:com.google.gwt.user.client.ui'>
  <ui:style gss="true" type="com.google.gwt.uibinder.test.client.gss.WidgetUsingGss.GssStyle">
    @def BLACK black;

    @defmixin size(WIDTH, HEIGHT) {
      width: WIDTH;
      height: HEIGHT;
    }

    .main {
      background-color: BLACK;
      color: white;
      @mixin size(200px, 300px);
    }
  </ui:style>
  <g:SimplePanel styleName="{style.main}"/>
</ui:UiBinder>