src/main/java/org/efaps/ui/wicket/components/tree/SortHeaderColumnLink.java
@Override
    protected void onComponentTag(final ComponentTag _tag)
    {
        super.onComponentTag(_tag);
        final UIStructurBrowser structurBrowser = (UIStructurBrowser) getDefaultModelObject();
        if (structurBrowser.getSortDirection().equals(SortDirection.ASCENDING)) {
            _tag.put("class", "sortLabelAsc");
        } else {
            _tag.put("class", "sortLabelDsc");
        }
    }
@Override
    protected void onComponentTag(final ComponentTag _tag)
    {
        super.onComponentTag(_tag);
        final UIStructurBrowser structurBrowser = (UIStructurBrowser) getDefaultModelObject();
        if (structurBrowser.getSortDirection().equals(SortDirection.ASCENDING)) {
            _tag.put("class", "sortLabelAsc");
        } else {
            _tag.put("class", "sortLabelDsc");
        }
    }
On the tag the class is set.

@param _tag tag to be set