WordPress at a glance
action is not described

setup_theme action-hook . WP 2.6.0

Fires before the theme is loaded.

Usage

add_action( 'setup_theme', 'action_function_name_5728' );
function action_function_name_5728(){
	// action...
}

Where the hook is called

In file: /wp-settings.php
setup_theme
wp-settings.php 407
do_action( 'setup_theme' );

Where the hook is used (in WP core)

wp-includes/class-wp-customize-manager.php 367
add_action( 'setup_theme', array( $this, 'setup_theme' ) );