If a user is on uportal and wanted to change the skin do the following.
Lookup the skin key in the skinList.xml
THen when on a maximized url in portal, open up your dev console and run
up.jQuery.ajax({url:'/portal/api/layout?action=chooseSkin&skinName=uwExtension', method: 'POST'});
Replacing uwExtension
with the skin key you want.
Then refresh your page.
good call