functions.php

<?php
function theme_enqueue_styles() {
    $parent_style = 'twentythirteen-style';
    wp_enqueue_style( $parent_style, get_template_directory_uri() . '/style.css' );
    wp_enqueue_style( 'twentythirteen-child-style',
        get_stylesheet_directory_uri() . '/style.css',
        array( $parent_style )
    );
}
add_action( 'wp_enqueue_scripts', 'theme_enqueue_styles' );
?>