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While developing a theme, I find it helpful to enable Smarty Debugging in "System" -> "Preferences" -> "General Settings".
When enabled, every page will add a popup window detailing the smarty templates utilized in creating the current page, and a list of smarty variables available.
Variables defined by XOOPS are listed as <{$xoops_<varname>}>. In your case, try using the <{$xoops_isuser}> to determine if the viewer is logged in and <{$xoops_username}> to retrieve their login name.
From all of the community, welcome to Xoops. Your contributions will be welcomed!