Source

File: wp-includes/customize/class-wp-customize-custom-css-setting.php

public function __construct( $manager, $id, $args = array() ) {
		parent::__construct( $manager, $id, $args );
		if ( 'custom_css' !== $this->id_data['base'] ) {
			throw new Exception( 'Expected custom_css id_base.' );
		}
		if ( 1 !== count( $this->id_data['keys'] ) || empty( $this->id_data['keys'][0] ) ) {
			throw new Exception( 'Expected single stylesheet key.' );
		}
		$this->stylesheet = $this->id_data['keys'][0];
	}