All of the OpenSkills web sites use a bootstrap script. This script allows the user to select a "look" that best suits the environment they are working in. A particular "look" is a combination of script and CSS files that define how the website is rendered (on the screen and when printing) and how it behaves.
The concept of a script which can switch the look of a web page is discussed:
Scripts can manipulate the content of a page via the DOM (Document Object Model)