includes/class-freemius.php
function _activate_theme_event_hook( $slug_or_name, $old_theme = false ) {
            $this->_storage->previous_theme = ( false !== $old_theme ) ?
                $old_theme->get_stylesheet() :
                $slug_or_name;

            $this->_activate_plugin_event_hook();
        }
function _activate_theme_event_hook( $slug_or_name, $old_theme = false ) {
            $this->_storage->previous_theme = ( false !== $old_theme ) ?
                $old_theme->get_stylesheet() :
                $slug_or_name;

            $this->_activate_plugin_event_hook();
        }
function _activate_theme_event_hook ( $ slug_or_name , $ old_theme = false ) { $ this -> _storage -> previous_theme = ( false !== $ old_theme ) ? $ old_theme -> get_stylesheet ( ) : $ slug_or_name ; $ this -> _activate_plugin_event_hook ( ) ; }
Saves the slug of the previous theme if it still exists so that it can be used by the logic in the opt-in
form that decides whether to add a close button to the opt-in dialog or not. So after a premium-only theme is
activated, the close button will appear and will reactivate the previous theme if clicked. If the previous
theme doesn't exist, then there will be no close button.

@author Leo Fajardo (@leorw)
@since  1.2.2

@param  string        $slug_or_name Old theme's slug or name.
@param  bool|WP_Theme $old_theme    WP_Theme instance of the old theme if it still exists.
Saves the slug of the previous theme if it still exists so that it can be used by the logic in the opt - in form that decides whether to add a close button to the opt - in dialog or not . So after a premium - only theme is activated the close button will appear and will reactivate the previous theme if clicked . If the previous theme doesn t exist then there will be no close button .