How can I allow full screen viewing of embedded YouTube videos in an upfront theme?

I set up a videos page on the site and embedded individual YouTube videos using the </> button. My client is rather particular about how they look, so simply linking to the videos wasn't enough. We needed to embed.

When I was doing that, I learned from a lovely support chat person that I had to set the width to 100% instead of using the given size, and I had to remove allowfullscreen. Otherwise the code wouldn't take.

So far so good, because everything works, is sized properly, doesn't show additional videos at the end, and looks lovely. However, my client wants to be able to view the videos in full screen mode. I deleted that bit and now cannot put it back in. The theme takes that out when I try to save it in the code.