Yeah, in the latest release of Gooey (9.0.0) I moved the themes to separate GitHub repos. I found that the asset/image browser in Defold became quite messy from all of the images that were included for the three Gooey themes. Having the themes in separate repos avoid this problem.
You should always do this! The installation instructions for Gooey and most other extensions and libraries usually say something like:
“Or point to the ZIP file of a specific release.”
In your case you need to pick the release prior to the breaking change. That would be version 8.1.1: