The theme packages OpenACS we have now are mostly for site-specific theming (colors, logos, css, js) but these are not sufficient for programmatic modifications introduced by different JS toolkits. E.g. some tree renders are purely JavaScript-based, others are based on e.g. div or list structures.
Therefore, xowiki has a site-wide parameter "PreferredCSSToolkit", which can be used to switch in essence between the old YUI style to the newer boostrap3 style of rendering. There are certainly for newer bootstrap versions (4 and 5) some differences, such as different button classes, or differences in the icon management (e.g. dropped glyphicon library in bootstrap4(, grid changes, etc... but many things work as before.
To address this, the change [1] was introduced shortly before the release of OpenACS 5.10.0 to provide a basis for a common bootstrap version, supporting as well newer versions. Our take was to skip Bootstrap4 and to go for Bootstrap5 which offers more options for the icon library. One challenge is to provide an easy setup optionally using the CDN like what we have for Bootstrap3. But for now, it is not clear, how much resources we can invest here in the short-range.
The way to go is to extend the "PreferredCSSToolkit" with newer bootstrap versions and to use the abstractions provided by [1]. The provided dict has to be extended, but when this is done, the goal is to switch between these major bootstrap revisions without code changes.
[1] https://github.com/openacs/xowiki/commit/99e3331c64d0af4f26c2ff41ec9f707cf40af3ef
