I have published a Bootstrap 5-based Omeka S theme at the following link.

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

How to use this theme is introduced in the following article.

[Omeka S Theme Development] Published a Bootstrap 5 Theme for Omeka S
[Omeka S Theme Development] Published a Bootstrap 5 theme for Omeka S
ldas.jp

As noted in the above article, there had been a bug with the display style option. This bug has been fixed in the following commit.

fix: layout bug · ldasjp8/Omeka-S-theme-Bootstrap5@3b45627
[DEPRECATED] Moved to https://github.com/nakamura196/Omeka-S-theme-Bootstrap5 - fix: layout bug · ldasjp8/Omeka-S-theme-Bootstrap5@3b45627
GitHub

I hope this is helpful for those using this theme.