string $old_name, WP_Theme $old_theme )
Fires on the next WP load after the theme has been switched.
The parameters differ according to whether the old theme exists or not.
If the old theme is missing, the old name will instead be the slug of the old theme.
Callback functions attached to this hook are only triggered in the theme (and/or child theme) being activated. To do things when your theme is deactivated, use switch_theme.
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do_action( 'after_switch_theme', $old_theme->get( 'Name' ), $old_theme );