Source

File: wp-includes/class.wp-styles.php

public function add_inline_style( $handle, $code ) {
		if ( ! $code ) {
			return false;
		}

		$after = $this->get_data( $handle, 'after' );
		if ( ! $after ) {
			$after = array();
		}

		$after[] = $code;

		return $this->add_data( $handle, 'after', $after );
	}