So after researching a little more I found my answer inside the plugin.json of the quickstart-plugin.
Adding a staticDirs key inside the plugin.json like that adds the static dirs inside /assets/plugins/your-plugin-name/.
A couple of years ago there was also a lower adoption of html5 elements. Having one h1 tag per page is still a good Idea in general, but it is allowable to have one h1 tag per containing article tag as well. Since this theme does not seem to use tags like article and aside (that I've seen in my quick check) I agree that there should only be one h1 tag per page which contains the page title or headline. The theme structure is otherwise good, but could definitely use with some contextual revamping of quite a few divs.