Child theme is a great option if you are using a theme downloaded from WordPress.org or another theme provider.

A child theme needs to have:
– their own theme folder in wp-content/themes e.g. wp-content/themes/xxx-child
– style.css file
– functions.php file

style.css

/*
 Theme Name:   Child Name
 Theme URI:    http://example.com/my-child/
 Description: Child Theme
 Author:       Jane Dow
 Author URI:   http://glorious-site.com
 Template:     parentFolderName
 Version:      1.0.0
 License:      GNU General Public License v2 or later
 License URI:  http://www.gnu.org/licenses/gpl-2.0.html
 Tags:         light, dark, two-columns, right-sidebar, responsive-layout, accessibility-ready
 Text Domain:  my-child
*/

functions.php

Note that functions.php in child theme is loaded in addition to the parent’s functions.php.

Enqueue both parent’s and child’s style.

<?php
function my_theme_enqueue_styles() {

    $parent_style = 'parent-style'; // This is 'twentyfifteen-style' for the Twenty Fifteen theme.

    wp_enqueue_style( $parent_style, get_template_directory_uri() . '/style.css' );
    wp_enqueue_style( 'child-style',
        get_stylesheet_directory_uri() . '/style.css',
        array( $parent_style ),
        wp_get_theme()->get('Version')
    );
}
add_action( 'wp_enqueue_scripts', 'my_theme_enqueue_styles' );
?>

More info on Codex – Child Themes