Below is how internally OpenText Content Widget loads different resources from Content server and renders the widget.
Here, we can see csui-base.css is downloaded from csui.js using helper.js. jquery-ui.css which imports original.css and adjustment.css has majority of the Stylesheet.
For custom look and feel, we had downloaded entire %support\csui\lib\jquery.ui\themes\opentext% directory and made local copy of it. After that, adjustment.css can be customized using CSS overwrite rule. We could completely customize entire CSS using JQuery theme roller as well.