uiBinder: style a DisclosurePanel

What's the best way to style (I need to make the headers bolder) that DisclosurePanel?

<g:DisclosurePanel>
  <g:header>Menu1</g:header>
  开发者_运维知识库<g:VerticalPanel>
    <g:Hyperlink targetHistoryToken="Entrate">Entrate</g:Hyperlink>
    <g:Hyperlink targetHistoryToken="Uscite">Uscite</g:Hyperlink>
  </g:VerticalPanel>
</g:DisclosurePanel>

<ui:style>
        .gwt-DisclosurePanel .header {
            font-size:20px;
        }   
    </ui:style>

Anyway, what's the best practice for styling gwt standard panels, via uibinder?