Import bootstrap framework templates

I read the documentation of Vaadin Flow and verified that it is possible to import html, java script and CSS, although with some regulations, it is not any html code. However, I tried to import the bootstrap template that I attached, but I have not succeeded. Does anyone know if it is possible to do it?

I have tried to do it with the Vaadin Designer, but apparently for that it is necessary to activate the payment subscription of the Vaadin Designer, apparently it does not allow it with the free version.

Greetings.