The skin ruleform generate dynamically CSS based on the model configuration - the css file is generated as static content and cached for your access group. To extend the CSS, you can include additional CSS files to the skin.

AngularJS is included in 7.4 and there is an example of how to integrate AngularJS / React and Vue.js into your Pega application in the UI Gallery in Pega 7.4.

As far as bootstrap is concerned - this is a very large framework that provides a lot of the same functionality as dynamic layout - The issue with most of the CSS framework is that the CSS requires a specific HTML markup to include the correct classes - If you want to use bootstrap for a 12 column grid, you will not be able to use OTB components like dynamic layouts and will need to use custom non-auto section / controls which might makes it harder to maintain.

a subset of JQuery UI is included in every harness and is used to render some specific components - it loads the CSS provided by the framework and the HTML is generated directly by the library thus keeping it in sync. You can use any widget library and attach the CSS files to your skin.