The rule that prevents lost work.
THE PROBLEM
Editing a theme's files directly means your changes are overwritten when the theme updates.
All of them, without warning.
WHAT A CHILD THEME IS
A separate theme that inherits everything from the parent and holds your changes.
The parent updates; your changes survive.
WHEN YOU NEED ONE
Any time you edit theme files Any custom functions Any template modification
WHEN YOU DO NOT
CSS-only changes, which can go in the Customiser's additional CSS Changes made through theme settings
THAT FIRST EXCEPTION
For simple styling, the Customiser is sufficient and simpler.
HOW TO CREATE ONE
A directory with a style file declaring the parent, and a functions file.
Many themes provide one ready-made. Check first.
WHAT TO PUT IN IT
Modified templates, copied from the parent and edited Custom functions Custom styles
WHAT NOT TO DO
Copy the entire parent theme into the child.
Copy only the files you change.
