I totally recommend you to use Child Theme if you are modifying core code.

What is Child Theme?

A child theme is a theme that inherits the functionality and styling of another theme, called the parent theme. Child themes are the recommended way of modifying an existing theme.

Whu use a Child Theme?

1. Safe Updates
A child theme automatically inherits the parent theme’s features, styles, and templates. This allows you to make changes to your site using child themes without ever modifying the parent theme. When a new version of the parent theme arrives, you can safely update it as all your modifications are saved in the child theme.

2. Easy to Extend

A child theme built on a powerful theme framework allows a great deal of flexibility without writing a lot of code. You can selectively modify only the template files and functions that you need without going through other template files. You can add new functionality and much more.

3. Fallback Safe

When you are creating a complete theme you need to think about all the possible scenarios and code for them. However, when you are working on a child theme and you forget to code for something, then there is always the parent theme’s functionality available as the fallback option.

( Source : http://www.wpbeginner.com/beginners-guide/wordpress-child-theme-pros-cons/ )

How to Create a Child Theme

Download Applique Child Theme here -> http://support.daffyhazan.com/files/applique-child.zip

Child Theme File Structure

• style.css
• functions.php
• screenshot.png

Customizing Child Theme

You can change theme behaviour with theme editing via functions.php

Example Case :
Change word amount of your excerpt.

The snippet is :

function custom_excerpt_length( $length ) {
	return 5;
}
add_filter( 'excerpt_length', 'custom_excerpt_length', 999 );

So your full code will be like :

NOTE :

– Please backup your customizer setting before you activated the child theme. ( For more details, read this manual -> http://support.daffyhazan.com/online-docs/installation-6/#importing-exporting-customizer )