You can use third party BS themes. Just don't expect all of Wappler's tools to be available to you. Wappler can't support third party themes. They don't know how they are built and every third party theme is different.
Even using a third party theme you still need to at least know how bootstrap works. The theme manager will be of no help to you. It is for creating themes, not editing them. In most cases you will have to contact the theme creator for support on how to edit them.
There is lots of discussion on the community about custom themes and third party themes.
https://community.wappler.io/search?q=custom%20bootstrap
You will also want to check out this documentation. It's for BS4 but it's exactly the same for BS5.
Best is to just ask about what you are trying to do. There are no dumb questions. Even if it's a bootstrap question, someone here will help you.
But in my opinion, you will become much better at it by spending a few minutes to learn the bare minimums of Bootstrap and build from scratch than use a third party template. All you really need to know about bootstrap is that the hierarchy is Containers -> Rows -> Columns. That's really all you need to know. Wappler even makes it easier by not allowing you to do anything that is not valid.
Bootstrap is probably the easiest framework to learn. It's basic at the most basic level.
Just ask questions. 
