This could be done by using the Url Fetch and Html Service from Google Apps Script.
With UrlFetchApp you could do GET/POST to the Google Forms servers, get the response to manipulate it and finally serve it as a web page by using the Html Service.
The broad idea behind the above was "on the wild" from a time ago. If found mentions of a Google Forms add-on called FormFuse that I think that works this way and a WordPress plug-in that does something similar.
It's worth to mention that currently there are two versions of Google Forms, the old and the new, as they are called by Google. The new version are generathe by Google Closure Compiler and the compiled form uses the JSAction library. The old version uses regular html forms, for me, this version is easier than the newer, as there are some tutorials that were made based on this version, like https://morningstudio.com.au/blog/2013/06/how-to-style-google-forms/. This tutorial doesn't use Google Apps Script but shows the broad concept.
A visual walk-through will be too long for this format. Please learn as much as you can of Google Apps Script and post new specific follow up questions.