I don’t see any further questions on your previous thread asking for clarification?
It works as simply as I stated:
theme = "github.com/user/repo"
This assumes your site is a hugo module. So you would do beforehand:
hugo mod init "your-mod-name"
As to why you couldn’t get it to work, I wouldn’t know. Refer to Requesting Help to see what information you would need to provide for us to be able to help.