I do not know where the CSS is located or how it is referenced in the Papermod theme and I am afraid that I do not have the time to look.
Typically an author includes a head
partial template that calls the CSS in a Hugo project. Most likely that is the template that you need to override.
To do just that under the root of your project replicate the PATH and the contents of head.html
under themes/papermod/
in a directory called /layouts/
then whatever addition you make to this template it will override the original template of the Papermod theme.