If you're using the nightly or alpha version of Mastodon (from the main Git branch), the only functional Mastodon Bird UI version is under the branch dev-build-system-css-theme-tokens. There's no release or release candidate yet. While the modularization work is still in progress, the theme is already functional and much lighter thanks to the new build system.
https://github.com/ronilaukkarinen/mastodon-bird-ui/tree/dev-build-system-css-theme-tokens