You would navigate to this part of the phpbb website which is http://www.phpbb.com/phpBB/catdb.php?db=2 and then pick one of the categories you would like to look for a skin once you find a skin download it and unzip it then upload it to your $phpbb_root_path/themes folder and then in the ACP click on add from Styles admin and then install your theme that you wish and if you want to change so that the skin is global meaning that its the default skin navigate into the Configuration part of your ACP under general board settings now just set it to your new skin that you have just installed.