We have published an Omeka S theme using Bootstrap 5 at the following location.

GitHub - ldasjp8/Omeka-S-theme-Bootstrap5: [DEPRECATED] Moved to https://github.com/nakamura196/Omeka-S-theme-Bootstrap5
[DEPRECATED] Moved to https://github.com/nakamura196/Omeka-S-theme-Bootstrap5 - ldasjp8/Omeka-S-theme-Bootstrap5
GitHub

Based on suggestions we received, we added creation date/update date as shown below.

Additionally, as Omeka S version 4 has been released as shown below, we made it compatible.

Omeka S version 4.0.0
The latest version of Omeka S is now out: version 4.0.0. You can download version 4.0.0 from omeka.org as usual. There are a few headline features in this release: Resource page blocks are a new feature allowing the administrators of individual sites to configure what’s shown and in what order on the item, item set, and media show pages visible to the public. Changes that used to require editing CSS to hide sections of the page, or edit the theme itself to insert or move pieces of content, ...
Omeka Forum

However, since there were methods that could not be used in common between version 3 and version 4 during the update, we created separate releases for version 3 and version 4.

  • For version 3
Release Bootstrap5-3.0.0 · ldasjp8/Omeka-S-theme-Bootstrap5
Released version 3.0.0.
GitHub
  • For version 4
Release Bootstrap5-4.0.0 · ldasjp8/Omeka-S-theme-Bootstrap5
Released version 4.0.0.
GitHub

Going forward, we plan to primarily make modifications for version 4, but I hope this is helpful.