It is recommended to use a child theme if you want to override the code of a theme without breaking the original theme’s code. This way, you make sure that you can update the original theme without loosing your edits.
Note: Using a child theme is optional. You can override CSS and JS without using a child theme by going to Appearance → Customize → Advanced Options.
Scandinavian comes packed with a child theme so you don’t have to create one by yourself. Here’s how to install it: