Modify theme ​

Depending on how you installed the theme, it might be harder or easier to modify it.

Hugo module ​

Using this method, there won't be any file under themes directory. In order to modify the theme, you will have to copy the file you want to modify to the same directory under layouts directory.

For example, in order to modify the themes/hugo-theme-stack/layouts/partials/head/custom.html file, you will have to copy it to layouts/partials/head/custom.html and modify it there (copy the code from theme's repository). The same applies to assets and static directories.

Git submodule ​

If you installed the theme through Git / Git submodule, you can modify the theme file directly and see the changes in your local site.

However, you can not commit and push the changes directly since you don't have the permission to push to the theme repository.

You need to fork the theme repository and push your changes to your forked repository (change submodule's repository url). Then, you can commit those changes to your site repository.