I did add two plug ins but things had been fine. No issues, I added just Yoast and Fusion but did not use Fusion because I saw the message that changes would not save from using it if I went back to Cornerstone.
I am not using a CDN.
I did not do any custom css or touch the child theme.
I was checking out all the settings in Appearance / Customize but not sure what I changed and positive I didn't save any changes while looking at the settings. I am very new to Cornerstone - wish I spent more time getting up to speed in the demos!