Piranha CMS uses Bootstrap 4.1.1 out of the box. The question is just this, is there any opportunity to use other css framework? I mean to use it in client part of the project. I understand that it’s possible to customize WYSIWYG editor to use any classes from any css framework, but in preview in manager module of Piranha CMS one will not see correct page view as cms uses Bootstrap 4.1.1.

As Piranha is a decoupled CMS you can use any css framework you want in your client application. The wysiwyg editor in the Admin UI can be styled with custom style sheets, but the concept here is not to mimic 100% the front end application, rather give the editor a good hint of what he/she is doing.

When you click preview in the admin it will actually open a new page that contains an iframe with the actual site, so here the editor will see the actual layout of the content.

Best regards