The editor is now completely blank page (nothing appears). I've activated another theme and reactivated Blank Foundation 6 one and still not working (how to debug?)

Also I noticed there is a dannyweeks-blank-foundation-6-child folder (and from looking at the active theme info, it seems this is the one that is actually active?) ... Not quite clear from docs how to go ahead and customize theme here - are file edits such as CSS + JS to be made in the child or the parent? Another question here: https://octobercms.com/theme/support/dannyweeks-blank-foundation/custom-css