First select the categories in which you want to add the JS or CSS code. If you select as root category the CSS or JS code will be applied throughout the public site. In this case, even if you choose other categories and choose not others, these codes will be applied even in those that have not been selected.
Adding Style Sheets
To add style sheets or fragments of this in the CSS tab is a code editor in which you can enter fragments of style sheets.
You can also add URLs of CSS files (one per line). For example, to add the style sheet of the latest version of Bootstrap in the Add own CSS field, just enter https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css. https://maxcdn.bootstrapcdn.com is the Bootstrap CDN URL.
You can also add URLs of JS files hosted on your own website or on some CDNs. Again with the Bootstrap example, if you need to use the JS file of this framework simply add the URL https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/js/bootstrap.min.js in the field Custom JS Files.
As mentioned at the beginning, these fragments or URLs will be included in the selected categories or throughout the public site if you choose Root as container category.