How do I turn on theme_debug? I want to use theme_debug to check which elements of a site use which .tpl files.

I have tried turning it on by using the drush command:

drush @mysite vset theme_debug 1


Though I do get the response theme_debug was set to 1, this does not seem to work.

Would anyone know if there is a way to get this working?

(Drush it's self seems to be working ok as I can use drush @mysite cc all)
Drush commands to control theme debugging:

Drupal 7


To turn it on: drush variable-set theme_debug 1
To turn it off: drush variable-set theme_debug 0


Drupal 8


To turn it on: drush state-set theme.debug 1
To turn it off: drush state-set theme.debug 0