@Paul Hema: I don’t know any such extension or way to do it in pure MediaWiki+PHP. Possibly you have to create an extension (the hard way) or write some JavaScript code. In JavaScript, you can do something like: (not tested, you have to debug it)
if( wgUserGroups.indexOf( 'autoconfirmed' ) == -1 ) {
$( '#n-mainpage-description' ).style( 'display', 'none' );
}
This should hide the element “Main page” (identified by id="n-mainpage-description" in HTML) for users who are not in the group “autoconfirmed” (search $wgUserGroups in HTML to see the groups of the current user). Such code can be put in your MediaWiki:Common.js.
Preview of a theme (“skin” in MediaWiki language) is possible for each user: go in your preferences, tab Appearance and click “Preview” next to a skin. The available skins must be previously installed by the system administrator, and enabled in LocalSettings.php.