do_action( 'switch_theme', string $new_name, WP_Theme $new_theme, WP_Theme $old_theme )

More Information

switch_theme is triggered when the blog’s theme is changed. Specifically, it fires after the theme has been switched but before the next request. Theme developers should use this hook to do things when their theme is deactivated.

Theme functions attached to this hook are only triggered in the theme being deactivated. To do things when your theme is activated, use after_switch_theme.

Functions attached to this action hook receive one parameter: a string with the name of the new theme being activated.