The easiest solution is to set your wizard not to show the navigator buttons, then create a button of your own:
<p:wizard widgetVar="myWizard" showNavBar="false" ... >
<p:commandButton style="..." value="Back" icon="ui-icon-arrow-1-w"
iconPos="right" type="button" onclick="myWizard.back();" />
<p:commandButton style="..." value="Next" icon="ui-icon-arrow-1-e"
iconPos="left" type="button" onclick="myWizard.next();" />
You can also do this with jQuery and CSS, but in my opinion it is more difficult this way.
Edit :-
For Primefaces5.1 +
<p:commandButton style="..." value="Back" icon="ui-icon-arrow-1-w" iconPos="right" onclick="PF('myWizard').back()" />
<p:commandButton style="..." value="Next" icon="ui-icon-arrow-1-e" iconPos="left" onclick="PF('myWizard').next();" />