Layout problem in Attachments 4.1.1 in site upgraded from J3 to J4

[jeffas]

I am upgrading a Joomla 3 site to Joomla 4.
I have replaced Attachments 3 (for J3) with Attachments 4.1.1 (from https://github.com/jmcameron/attachments/releases/)
Attachments 3 has pop-up dialogues (iframes) for uploading and editing attachments.
I believe that Attachments 4 is supposed to operate the same way.

The problem is that all the iframes appear inline instead of popping up when the relevant link/button is clicked.

I think this is to do with various classes and attributes with the word "modal" in their name (e.g. class="modal-body", data-bs-toggle='modal').

I am using a J3 template ("Favourite" from FavThemes) which I have adapted to work with J4.
I think that is the root of the problem, because if I switch a page to Cassiopeia, the pop-ups work correctly.

Can anyone point me to what I need to add or change in the template to get this working correctly?

(I like the Attachments extension, and the look that I've got with the Favourite extension, so I really don't want to have to switch to different ones.)

[Per Yngve Berg]

You have to consider the difference between version 2 and 5 of Bootstrap.

[jeffas]

You have to consider the difference between version 2 and 5 of Bootstrap.

Thanks Per.
I managed to get the right behaviour by copying a lot of template.min.css from Cassiopeia into the Favourite template. I have probably included more than I need, but it is working now. There remain some minor layout differences, which may well be related to bootstrap, but I can live with them!