Using Child theme allows you to keep any changes / modifications, when you update your theme

1. You can download the child theme from your fancythemes dashboard. 
2. Install the child theme like you do any other WordPress theme. It will automatically inherit the parent theme settings. 
3. Activate the theme. Now any changes you make in your child theme, would override the parent theme.

Alternatively, you can use a custom CSS plugin, if you don’t want to go through the hassle of installing the child theme.

Additional Resources: 

• How to Install a WordPress Theme
• What is a Child Theme & How to Create