<replace-with class='com.acdlabs.Test_portal.web.theme.client.header.TestColumnHeaderAppearance'>
    <when-type-is class='com.sencha.gxt.widget.core.client.grid.ColumnHeader.ColumnHeaderAppearance' />
</replace-with>

<module>
    <inherits name="com.sencha.gxt.theme.base.Base" />

    <replace-with class='com.acdlabs.Test_portal.web.theme.client.header.TestColumnHeaderAppearance'>
        <when-type-is class='com.sencha.gxt.widget.core.client.grid.ColumnHeader.ColumnHeaderAppearance' />
    </replace-with>
</module>

public class TestColumnHeaderAppearance extends ColumnHeaderDefaultAppearance {

    public interface TestColumnHeaderResources extends ColumnHeaderResources {
        @Override
        @Source({ "com/sencha/gxt/theme/base/client/grid/ColumnHeader.gss", "TestColumnHeader.gss" })
        public TestColumnHeaderStyle style();
    }

    public interface TestColumnHeaderStyle extends DefaultColumnHeaderStyles {
        String filterIcon();
    }

    public TestColumnHeaderAppearance() {
        super(GWT.<ColumnHeaderResources> create(TestColumnHeaderResources.class));
    }

}

public class TestColumnHeader<M> extends ColumnHeader<M> {

    public TestColumnHeader(Grid<M> container, ColumnModel<M> cm, ColumnHeaderAppearance appearance) {
        super(container, cm, appearance);
        if (appearance.styles() instanceof TestColumnHeaderStyle) {
            styles = (TestColumnHeaderStyle) appearance.styles(); // <-- this is never reached and I that's why I get exceptions like "Cannot read property 'filterIcon_0_g$' of undefined"
        }
    }
}