Right now looking at the reference of Google Forms API, I cannot find any relevant item for modiying a Form's theme like this UI:
Did I miss anything? If the API is really missing, where can I request it?
Apparently this feature is missing and is tracked here but the last update was 6 years ago.
At the moment it is not possible to change Google Forms theme through the API. The Google Forms API still works with the first version of Google Forms, for the moment there are no themes or text styles available programmatically.
To request features for the Google Forms API, you can do it through here. If there is already a request then you can add a comment to the thread. This is another feature request related to what you're asking.