I am new to Hugo, have been looking at the docs for some time, trying to create a library-site.
What if I found a theme that is a good foundation for me but I need to modify most of it e.g. I do not want to have “posts” but “books”, different colors, different font, a bit modified layout for textual data and header/footer, modified taxonomies… I know that I can override nearly everything but is it worth it, is that a good practice in this case? Why not just copying the theme (credit should be given to original authors) and throwing out files / editing sources?
Depending on the theme you might be able to make use of theme components to override specific parts of the layout (like headers, footers or single page templates).
Also, usually you can configure taxonomies from your site config and any theme should work with them or have some extra configuration options for that
For very small and specific changes you could just add stuff directly to your layouts folder to override corresponding files in the theme.
You also have the option to create your own theme. I used the getting started videos, the Docs, and the Community as resources to create my theme. I used tachyons.io for styles and html snippets.