Go into your config.php file located in the geeklog root directory and edit line 138 or value to "$_CONF['theme']" and enter in the name of your theme there.
i.e. if !1_White_Pointer was the name of your theme...
Text Formatted Code
i.e. $_CONF['theme'] = '!1_White_Pointer'; // default theme
This is for Geeklog 1.3.8 and will make the theme you specify there the default theme for anonymous users.