Welcome to Geeklog Monday, May 20 2013 @ 10:47 AM EDT
The default theme that Geeklog uses for anonymous and for new users is set in config.php:
$_CONF['theme'] = 'XSilver';
The name of the theme is the name of the theme's directory below Geeklog's layout directory. Please note that this name is case-sensitive on case-sensitive file systems (e.g. most variants of Unix / Linux).
Users can change their preferred theme in their preferences, where there is a drop down menu of all the currently available themes. Again, this simply lists all the directories below the layout directory, so adding or removing themes from this list is done by adding or removing the theme directories.
If you don't want your users to be able to change the theme, you can either remove all the unwanted themes or you can disable the option to change the theme by setting
$_CONF['allow_user_themes'] = 0;
in your config.php.
If Geeklog can't find a theme, it will revert to the default theme, so it's safe to remove themes at any time, as long as the directory for the default theme exists.
Once a user changes his/her default theme, Geeklog will also set a cookie with that theme, so that they still see the site with their preferred theme, even when they're not logged in.