put an IF statement in the template files like this
eg for menu.tpl
Code:
{if $usertype eq "A" or ($usertype eq "P" and $active_modules.Simple_Mode ne "")}
put your admin style menu code here
{else}
put your customer style menu code here
{/if}
You can use this method throughout the templates that are shared between admin and customer.
From v4, admin and customer have different stylesheets.