SXA 9.3 was released with Sitecore 9.3 at the early of this month. They not just changed the new, also provide a new way to handle the theme.
Use CLI tools is a new way to deal with the theme.
An article about how to install the new CLI tool can be found at https://doc.sitecore.com/developers/sxa/93/sitecore-experience-accelerator/en/add-a-theme-using-sxa-cli.html
There is also a quick demo - https://www.youtube.com/watch?v=MDWuqhmvwgI
You will need to escape the @ character if you are using powershell:
npm config set `@sxa:registry https://sitecore.myget.org/F/sc-npm-packages/npm/