Tonight I used bin/plugin devtools new-theme to make a copy (machineCopy) of the Machine theme which I had significantly modified.
When I switched to the copied them I just got an essentially blank page with a statement that the theme machineCopy doesn’t exist.
After digging around I saw that almost everywhere “machine” had been changed to “machineCopy”. The one place I found where it wasn’t changed was in the block below.
After I changed user/themes/machine to
user/themes/machineCopy, it worked fine.
# Grav streams
streams:
schemes:
theme:
type: ReadOnlyStream
paths:
- user/themes/machine