With the Beautiful Hugo theme:
- Single pages are rendered by themes/beautifulhugo/layouts/_default/single.html
- The home page is rendered by themes/beautifulhugo/layouts/index.html
Both page kinds are wrapped by themes/beautifulhugo/layouts/_default/baseof.html.
To override these templates, create a template with the same path in the layouts directory in the root of your project. For example, to override themes/beautifulhugo/layouts/_default/single.html, create layouts/_default/single.html.
Do not edit files within the themes directory.
I suggest you experiment with the templates described above, then come back here with specific questions.