Child Theme
If you need to modify core files or just need a better way to manage custom CSS, we recommend using the child theme.
The fastest way to install the child theme is by going to Appearance -> Themes -> Add New Theme & Activate Child theme.
Now you have the child theme actived, you can start add your custom CSS into the child theme style.css file, or your custom PHP code in functions.php file, via Appearance -> Editor.