Prismatic is a parent theme. If you want to change its styles or templates, create a child theme. Your changes live in the child theme, so updating Prismatic never overwrites them.

Create the child theme

1. In your wp-content/themes folder, create a new folder called prismatic-child.
2. Inside it, create a file called style.css.
3. Add the following to the top of style.css:

/*!
 * Theme Name: Prismatic Child
 * Theme URI: https://link-to-your-site.com
 * Description: Describe what your theme should be like.
 * Version: 0.1
 * Author: Your Name
 * Author URI: https://link-to-your-site.com
 * Template: prismatic
 */

Template must be the parent theme’s folder name, prismatic.

Activate it

Go to Appearance → Themes and activate Prismatic Child. The parent theme’s styles load automatically, so your site looks exactly the same. Now add your own CSS below the header in style.css, and it will be applied on top of Prismatic.

Keep Prismatic installed

The child theme depends on Prismatic, so leave Prismatic installed, but not active, and keep it updated.