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
If you have a fediverse account, you can quote this note from your own instance. Search https://mementomori.social/users/rolle/statuses/115826155675077541 on your instance and quote it. (Note that quoting is not supported in Mastodon.)