Combobox elements not customizable with css class

Combobox's elements (content) are rendered in html as a div appended at the very end my html document. The class give to the combobox (and to its ancestor nodes) are not available to the generated div. For example:

<div style="..." class="z-combobox-pp z-combobox-shadow">
	<table ...>
		<tbody>
			<tr class="testClass z-comboitem" id="n28Qp6">
				<td class="z-comboitem-img"></td>
				<td class="z-comboitem-text">Demo</td>
			</tr>
		</tbody>
	</table>
</div>

Therefore I cannot customize look and feel of my list.