You can also add jQuery to your site via Settings > Code Injection > Header. Then place the following into it:
Putting this into your site on Squarespace will allow you to use jQuery on your site. It can also be placed on individual pages if you so need it.
CSS is best placed onto the individual page. If it is site wide you can add it to your code injection as a reference. Make certain to check via browser inspector to insure that your external reference is not running into errors such as not being found and what not.
Once you have all the elements in place you can then reference them via Code Block within your page.
Brief instructions on how to add jQuery on my site.
Hope this helps!