Many of you are probably using the flagship XOOPS theme, xSwatch.
This theme is based on Bootswatch which is itself based on the famous Bootstrap CSS library.

The version currently available for XOOPS is xSwatch4.
This theme uses Bootstrap version 4.6.1.

Since then, Bootstrap has released a version 5, and more precisely 5.2.3, the latest stable version to date.
Our goal is therefore to offer web builders using XOOPS, the xSwatch theme with Bootstrap version 5.

The migration work is however consequent.
Indeed, there are several changes. Here are just a few of them.

• First of all, elements with "left"/"l" and "right"/"r" will no longer be processed, but you will have to use "start"/"s" and "end"/"e" on all elements that used them.
  Why this change?
  To have more clarity for countries where the writing goes from right to left (RTL).
  Thus, whatever the language system used, we will have "start" and "end"!
  Example: mr-2 becomes me-2
  Impact: This obviously affects elements such as Padding, Margin, Alignment and border.
  
  
• There are changes in the forms
  
  
• All calls to Javascript are now made not via data but via data-bs.
  Example: data-slide becomes data-bs-slide
  Impact: carousel, hamburger menu, dropdown menus in particular.
  
  
• And then, in bulk, there are impacts on the alert, navbar, and grid layout elements.

It is thus necessary to take again point by point the whole of the files which compose xSwatch. A development repository has been opened here:
https://github.com/alain01/xswatch5

We invite the community to participate to this migration work.
In the README.md file, you will find the "TO DO" list on what remains to be migrated.

Finally, we will take advantage of this migration work to propose you at the end an article of migration or a tutorial.

Alain

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References

Migrating to v5
Cheatsheet on converting Bootstrap v4 project to Bootstrap v5
Migrating from bootstrap version 4 to version 5
Migrate from Bootstrap 4 to Bootstrap 5
How to Migrate from Bootstrap Version 4 to 5