Safely using CSS/JS libraries
PowerUI is now at the point where it can make use of a range of existing libraries. These libraries are typically available for the web, so the classic method to include them is via a link to a remote site:
<!-- How it's normally done on the web -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.0/jquery.min.js"></script>
Don't do this in PowerUI. You're creating an internet requirement where it's often entirely unnecessary. Just imagine if somebody found they couldn't run your app on a plane, for example, despite it appearing to be offline.
Add local copies to your project
Drop in the .css and .js files into any folder (or sub-folder) of any Resources folder in your project - here's some examples:
Assets/Resources/CSS/myStyle.css.bytes Assets/UI's/Resources/jQuery-min.js.bytes
You can have as many Resources folders as you want/ need to organise your project as you wish.
Then rename them so Unity doesn't think .js is UnityScript (Unity completely ignores .css too). We recommend adding .bytes onto the end as that way future changes to Unity won't break your naming schemes.
Referencing them
By default, PowerUI's custom resources:// protocol is used. So, the path you use is relative to the parent Resources folder, minus .bytes:
<!-- Local referencing in PowerUI - internally uses resources:// -->
<link href='CSS/myStyle.css'></link>
<script src="jQuery-min.js"></script>