I am not sure but I think that the 2018 Cornerstone may have added a bit of complexity to the process. It appears that the design is different from the 2017. I had actually installed them on our 2017.
I hope these two photos show the diagonal trim covered by carpet type material. The frame and the square tubing are behind it. (I guess) I also don't see a way to lower the trim.
I'm not very handy so I apologize to those of you who are.
Thanks in advance
Note: The second photo is actually one photo but the light bar makes it look like two!