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Bennie,
The XOOPS core does not directly support the notion of different style-sheets for different groups of users.
You could probably build this into a theme that linked in different styles depending on whether the user was registered or not.
What you said that indicated to me that this was not what you wanted is that you want to change the layout. The defined mechanism in XOOPS for dynamically changing the layout based on the user is group permissions.
Xoops allows you to assign permissions to the anonymous group with regard to modules and blocks that differ from the registered group so that the content and layout appear to change depending on which group you are in.
Example, I have built sites that reserve the right block for administrator and moderator information, so anonymous and registered users have very simple, clean looking pages. I also remove access to blocks that are more member oriented (top posters, new members, etc.) from the anonymous group.
Because XOOPS layout is dynamic, based on which blocks are assigned to which screen region (left, leftcenter, right center, center, right) then you can control where blocks appear and which ones appear for which user groups.
If you wanted content to appear in different positions based on who the user was, then you would require a hack, like cloning blocks and having one clone assigned to one user group in one region, and the other clone to a different user group in a different region.
Hope this explanation helps...