Add the javascript to your templates for the first one, and study theme components to combine two themes, for the second. In any case, read all the docs.
The particles.js github shows how to integrate it. You need to search about:
overriding theme components - find the appropriate template, copy it to your project from the themes folder to the same folder in your project, and edit it
hugo’s static folder - that is where you can put js files like this one
As for combining themes, that’s not a trivial task, so I cannot imagine anyone simply volunteering to figure that out for you. I would suggest getting more experience in how Hugo works, then start doing it yourself. And, read all the docs as @maiki mentions.