Create a Child Theme

Choose a Theme and make a Child Theme:


Locate and install a starting point theme:

https://wordpress.org/themes/

Once installed, create a child theme based upon your chosen theme.

http://codex.wordpress.org/Child_Themes

You use a child theme so that you can freely modify the theme, yet still be able to update the parent theme without wiping out all your personal modifications.
The fasted way to set up a child theme is to install the “one click child theme” plugin and click on the new child theme option that then appears on the installed theme.
The manual way to create a theme is good to know how to do and is as follows:

In the same folder as your parent theme, create a folder with the same name as your parent theme with “-child” at the end. So if your main parent theme is TwentyFifteen, then your child theme would be TwentyFifteen-child. In that folder you will put at least two files, a styles.css and a functions.php. The styles.css is to have the following header information.

/*

Theme Name:   Twenty Fifteen Child
Theme URI:    http://example.com/twenty-fifteen-child/
Description:  Twenty Fifteen Child Theme
Author:       John Doe
Author URI:   http://example.com
Template:     twentyfifteen
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:  twenty-fifteen-child
*/

/* Your styles come after this */

The important header item is the “Template: twentyfifteen” as it is how WordPress knows to connect it to the twentyfifteen theme as its parent theme.
You will need to create a functions.php file that has the following code for the new (best practice) way of enqueuing the child theme’s styles.css file.

<?php

function theme_enqueue_styles() {

$parent_style = 'parent-style';

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 ));
}

add_action( 'wp_enqueue_scripts', 'theme_enqueue_styles' );