After investigating GeneratePress and Elementor, I decided to take a risk on using the two together. I was in a rush and thought Elementor would give me a leg up and then I could build using the GeneratePress theme when I had more time and needed more control.
The experience with Elementor Pro has been good. I like the workflow that it supports. But I have run into a significant shortcoming of the tool not supporting fixed backgrounds on mobile devices. I want to go behind the scenes to create a work-around. I have GeneratePress premium. Could you point me to where where the Elementor code is and how to go about customizing the resulting GeneratePress theme without breaking things.