How one can switch/change PHPBB theme/style while one do not have theme files, but theme is mentioned in MySQL?
A) change theme name in mysql (complicated)
B) copy name of the theme that is missing (enter your forum and you got error like:
OK, go to Your hosting File manager and navigate into "styles" directory. Copy one style folder and name it as an missing style ***.General Error
Template path could not be found: styles/***/template
Try to refresh your forum page and access admin area to "Purge cache" (button on the main screen after admin login). If not able to login admin area, go back to File manager and backup "cache" folder, then delete from this folder all files except .htaccess and index.* file and refresh your forum main page..
Then login and switch your PHPBB forum theme & delete the redundant theme folder from File manager, "styles" folder.