How to hide or rename “X” and “x-child” references in website source?

In the child theme style.css file, change the following info:

Theme Name: X Child Theme
Theme URI: http://theme.co/x/
Author: Themeco
Author URI: http://theme.co/
Description: Make all of your modifications to X in this child theme.
Version: 1.0.0
Template: x

Change the Bold items above to whatever you want, just make sure to keep the Template reference as is.

You can also change the screenshot.png file in your child theme folder to reflect an image you’d like to display, should someone look up what wordpress theme you are using- with a website like whatwpthemeisthat.com or something similar.