I published a Bootstrap 5 theme for Omeka S.
https://github.com/ldasjp8/Omeka-S-theme-Bootstrap5
Below is an explanation of how to use it, including information for developers.

The following settings are available on the theme settings screen.

I will explain some of the settings above.
Footer Content
Enter the text to display in the footer. You can also enter HTML as shown below.


Site Sub Title
When a subtitle is set, it is displayed on the top page as shown below. Note that this feature only works when the “Next” module is enabled.
https://github.com/Daniel-KM/Omeka-S-module-Next

Top Image
Select an image to use as the background image on the site’s top page. Note that this feature only works when the “Next” module is enabled.
https://github.com/Daniel-KM/Omeka-S-module-Next

The relevant source code is as follows.
Also, as shown below, the isHomePage function is only available when the Next module is enabled.
Therefore, if the Next module is not installed and the URL of the page used for the top page is known, it is also possible to use a custom function that assigns a value to the $isHomePage variable.
Top Button URL
When a URL is set, a “Browse” button that navigates to the configured URL is displayed on the top page as shown below.

Sort Properties
Enter the properties to use for sorting.
As shown below, set the Term and label separated by commas.

With this setting, you can confirm that the configured content is reflected as follows.

The theme customization details can be found here.
Body Properties
Enter the properties to use on the search results list page.
As shown below, set the Term and label separated by commas.

With this setting, you can confirm that the configured content is reflected as follows.

The theme customization details can be found here.
The following shows the part that loads theme settings.
The following shows the part that modifies display content based on the loaded settings.
Layout for Browse Pages
Set the default display style.
(As of March 14, 2022, due to a bug, it does not work properly when “List” is set. A fix is planned.)
Show a Link to Collections in Item Pages?
Configure whether to add a link to item sets on the item detail page. Set to “No” when using a single item set, for example.
Other
Apart from theme settings, you can change the “Title” and “Summary” on the site settings page.

As a result, the configured content is reflected as follows.

Note that these are also used on the site list page.

Summary
This theme still has many bugs and missing features, but I plan to improve it gradually. Please let me know if you have any requests for fixes.