Google forms are presented for the user to embed in his/her website as an iframe, which means they can't be styled by the website's own CSS.
How can a Google Form be inserted into a website with a custom style using "new" Google Forms?
A similar question exists for which there is an answer for the old style Google Forms, which involves copying the source HTML form generated, however this does not work for new (post 2015) forms.
If there's no way to do this, please suggest some alternative, such as using Google Apps Script to host a page that can be styled, then embedded to a website - or some other solution.