One of the new features of the Mendix 5, is being bootstrap compatible, is that right? There are many bootstrap theme available on the internet https://wrapbootstrap.com/ Trying to implement some of these themes, gives me noting that looks like the sample pages. Is there a way to make it work? Is Mendix 5 generating clean bootstrap html code, so others themes can be applied? Which version of bootstrap is Mx5 Beta using? Recently bootstrap 3 was release, will that be part of Mx5 too?
The upcoming release of Mendix 5 comes with Bootstrap 3, though up to beta-8 Bootstrap 2 is used. Bootstrap 3 is not backwards compatible so Bootstrap 3 themes can not be used with Mendix 5 beta-8 and before and vice versa. That may explain your theme issues.
Beware that as of version 3, Bootstrap encourages the use of a separate theme file, instead of replacing bootstrap.css with a new one. See the default index.html for an example of how a theme should be included.