Nailed the code into a single query.
PHP Code:
SELECT `ft`.`slug`, `ft`.`title` FROM `forum topics` AS `ft` WHERE `ft`.`theme` IN(SELECT `t`.`id` FROM `users themes` AS `ut` LEFT JOIN `themes` AS `t` ON `ut`.`themeid` = `t`.`id` WHERE `ut`.`userid` = '{$user->id}') ORDER BY `ft`.`date` ASC