WordPress at a glance
action is not described

after_setup_theme action-hook . WP 3.0.0

Fires after the theme is loaded.

Usage

add_action( 'after_setup_theme', 'action_function_name_6351' );
function action_function_name_6351(){
	// action...
}

Where the hook is called

In file: /wp-settings.php
after_setup_theme
wp-settings.php 451
do_action( 'after_setup_theme' );

Where the hook is used (in WP core)

wp-includes/class-wp-customize-manager.php 566
add_action( 'after_setup_theme', array( $this, 'after_setup_theme' ) );
wp-includes/class-wp-customize-manager.php 586
add_action( 'after_setup_theme', array( $this, 'establish_loaded_changeset' ), 5 );
wp-includes/class-wp-customize-manager.php 594
add_action( 'after_setup_theme', array( $this, 'import_theme_starter_content' ), 100 );