If you examin some of the free skin packs on various sites around the community including mine, you can check out the skin.css and skin.xml to see the way the solpartmenu styled.

Your second question, either put solpartmenu in a table and let it be in the centre. or use dnn:SOLPARTMENU runat="server" MenuAlignment="" /> set the menu alignment to left/middle/right

if HTML type skinning then you need to put it in skin.xml

   <Setting>
    <Name>MenuAlignment</Name>
    <Value>Middle</Value>
   </Setting>