Basically, you just put the HTML for the template in the Manager (Manage Resources, Templates tab), and upload all the files, css and images related to the template in the /assets/templates/TemplateName folder (where TemplateName is, of course, the name of your template). Just make sure the links in the <head> part of the HTML code point to the proper path.
Set the template you want to use by default in the Configuration (Tools -> Configuration), and you can set each document’s template when you create or edit the document. I believe that there is also a plugin to cause documents to inherit a parent’s template automatically.
Other than that, there is the
SkinGraft module, which I use on my Loripsom template demo site. The number of templates available for it is increasing as I find the time to package up new ones.