Hi Rob,
Technically either from discord or from this forum, downloading the latest dev (development) build is the best choice... Beta.5 is ok too but it had some nice bugfixes after release and latest dev is better than the beta.5 at the moment.
Additionally, when you duplicate a theme (be it the default theme or an addon theme), you need to (this is a must) edit the theme.json and change the name, which needs to match the folder name you used. Only then it will be possible to use it and avoid any errors like you mentioned above.
So considering the error in your hands; You need to either clean the application cache from admin > maintenance page or if you can not reach there too, then you need to delete the mentioned (bootstrap/cache/themes.php) file manually for it to be re-created with the corrected info.
Hope this helps, good luck