No, I definitely understand the problem you’re having. I’ve had it too, and there are several threads about this issue on the forum.
There are a number of different things that can lead to this issue, but at core the problem is that wkhtmltopdf is not able to access your css file locally. It usually takes some troubleshooting to fix, but explicitly setting your domain is often one of the necessary steps. Updating your /etc/hosts file is also sometimes necessary.
Unfortunately, wkhtmltopdf is a fussy creature, and it’s created a challenge for many.