I have published a Bootstrap 5-based Omeka S theme at the following link.
https://github.com/ldasjp8/Omeka-S-theme-Bootstrap5
How to use this theme is introduced in the following article.
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.
https://github.com/ldasjp8/Omeka-S-theme-Bootstrap5/commit/3b456277fe4f75cdcc0044a0fe0df7e7e2e14156
I hope this is helpful for those using this theme.