About the customization of login / signup / password reset pages in Customer Portal.

I am trying to reflect the clients brand, but there seems limited options. How to change the styling of the included forms. Eg default light blue colour of titles and buttons?

I am confused by this post https://help.zoho.com/portal/en/community/topic/customer-portal-improvement

where I see screenshot of highly customized signup page. 

This is my interface attached. Can I load additional CSS somewhere in the page? Or is there other access to change the styling