The Developer tab allows for the introduction of minimal custom code modifications, that range from CSS to JavaScript.
Custom CSS
This zone, allows you to add your custom CSS to the page
Custom Script
Allows the addition of custom scripts to the page.
Note: This zone will load after the scripts added in the addicional resources zone.
Additional Resources
Allows you to add CSS or JS files to your theme. These files load synchronously, in the order you have inserted them.