Hey @ATA-Daniel
I apologize for the delay in answering this post, were you able to find the answer for it?
If not, at the moment as you can see in our Knowledge base article: Pop-up forms and non-HubSpot forms FAQ it is not recommended to add your own style to these forms. However if you were to add your own style you would need to create a custom-coded solution for this.
I hope this helps,
Kindly,
Pam