Maybe you can take a look at nodebb-theme-lavender, it is a child theme based on vanilla in nodebb 2.x and persona on nodebb 3.x
nodebb 2.x https://github.com/NodeBB/nodebb-theme-lavender/tree/v6
nodebb 3.x https://github.com/NodeBB/nodebb-theme-lavender/tree/master
In theme.json the baseTheme is set to the parent theme "baseTheme": "nodebb-theme-persona" and in the main style file the scss/less of the parent theme is imported. https://github.com/NodeBB/nodebb-theme-lavender/blob/master/theme.scss
If you share the error messages you get we can help more.