I am having some issues trying to style my form for mobile. I know very little, but i think it may be something to do with the container? The page it is on can be seen here.
In your Design Manager you should be able to find a file called parent-site-style.css. If you Ctrl + F to search, you should find something like the below:
If you add “!important” after the width, as below, and Publish the file, it should update your form. This forces it to override any previous formatting.