In XOOPS 2.5.11, there are 2 major changes that have an impact on your existing sites:

• A new version of the template engine, Smarty 3, whereas previous versions of XOOPS used smarty 2,
• The possibility of using PHP 8.x - highly recommended - which provides security updates and removes functions obsolete under PHP 7.

These 2 developments can have a major impact on the templates used, both the default ones in modules and those you use as overloads.

Like any good webmaster, you're thinking, “Haaaa, I'm still on XOOPS 2.5.10 (or even lower), I need to upgrade to XOOPS 2.5.11”. So, since you're such a pro, you're going to set up a test site to see the impact of this migration. This test site will be :

• Either a sub-domain of your domain, on your hosted server,
  Example: sandbox1.mydomain.com
• Or locally
  and in this case, under MS Windows, you can use https://bearsampp.com/ (read the article Bearsampp, Bearsampp, the current fork of Neard) by following the tutorial Using WAMP Neard with XOOPS (Follow this tutorial using Bearsampp).

Then,

• Either test a blank installation of XOOPS 2.5.11 and add your theme from your previous version
• Or make a copy of your site (files + database).

In the second case, you'll need to follow the upgrade procedure, which you'll find in the XOOPS 2.5.11 repository, in the /upgrade/README.md file.

To fix problems with older themes, you'll need to proceed methodically.
It may sound complicated, but it's actually simpler than you might think.

The most important points:

1. Make the “Themes” block visible from the Control Panel / BLOCKS.
   
   
2. In Preferences / General Preferences :
   • Choose for Default theme: xswatch4
   • Choose from Selectable themes: my-theme + xswatch4
   This will allow you to revert to a working theme in the event of a blank page with the my-theme theme to be migrated.
   
   
3. Be sure to read the tutorial Upgrade smarty 2 to smarty 3/4
   
   
   To simplify debugging

   We have noticed that the error messages are not easily exploitable if you are with a version 8.1.X of php or higher.
   To simplify debugging, you need to enable PHP version 7.4.X or 8.0.X.
   This is very important, otherwise you will not be able to interpret the error messages and this will make it very difficult to fix.
   
   Another essential point is to activate the debug mode of XOOPS to read the error information!
   Control panel / Preferences / General settings / Enable debug : "Enable debug (OnLine mode)"
4. Update the modules you use
   
   
5. Template overloads
   
   • If your my-theme theme uses Bootstrap 3, then you need to retrieve the XOOPS 2.5.11 distribution overloads located under
     /themes/xbootstrap/modules and copy them to themes/my-theme/modules.
   • If your my-theme theme uses Bootstrap 4, then you need to retrieve the XOOPS 2.5.11 distribution overloads from
     /themes/xswatch4/modules and copy them to /themes/my-theme/modules.
   • If your my-theme theme uses neither Bootstrap 3 nor Bootstrap 4, then you'll need to retrieve the templates of the modules you're overloading from the folders :
     /modules/*modules_whose_templates_are_overloaded_in_my-theme*/templates
     and copy them to /themes/my-theme/modules/
     
     
   Obviously, copying these overloads will overwrite your overloads, which were probably not operational for PHP8.
   You have the option of comparing templates rather than overwriting them, but this is rather time-consuming.
   
   
   
6. Once all this has been done, you can go back to the Smarty tutorial and correct any problems you've encountered with the help of the Error Handling page.
   
   
7. Once your mon-theme theme is stable, remove the “Themes” block and choose “mon-theme” as your default theme.
   
   

Now you've done it!
Finally, use the forum if you get stuck.
Don't hesitate to share your experience here too.

All the best!