you don't have to clear the templates_c folder go to admin -> system admin -> preferences -> general settings -> turn "Check templates for modifications ?" to YES /*when you edit the theme, I do not know if this works for changing the templates*/
please first search then ask https://xoops.org/modules/newbb/viewtopic.php?topic_id=63951&forum=32&post_id=288451#forumpost288451 or https://xoops.org/modules/newbb/viewtopic.php?topic_id=62253&forum=32&post_id=280919#forumpost280919