Hi Edwin,
OACS 4.5 and above has templating system. Its a bit different from phpnuke since OpenACS is more catered toward developers and bigger sites.
I don't have experience on phpnuke but there is a good change that the templating system of OpenACS 4.5 is more powerful than PHP Nuke.
This templating system is used by the subsite template. Its also used on some other areas but I will use the subsite as an example. Each OpenACS site can have 1 or more subsites. That would mean each subsite can have a different look/theme. In one of our sites each client has a different subsite and look. So the OpenACS site has about more that a dozen themes in it.
I hope you get encouranged to try out OpenACS. If you are looking for a real powerful workhorse then OpenACS is one of them.
