v13 uses a pdf generator called wkhtmltopdf. It’s a nightmare to work with, and there are dozens of threads about it on this forum. The short answer is that wkhtmltopdf can’t access your css and other assets, likely due to a permissions or addressing issue.
Unfortunately, there’s not much Frappe can do in the application space to solve this issue in the application space, as wkhtmltopdf is installed separate binary executable. It’s an issue of server configuration. For what it’s worth, v14 has ditched wkhtmltopdf for weasyprint for exactly this reason.