Well, I disabled all themes from PrimeFaces:
primefaces.THEME
none
Almost all other components will fail to, look and feel wise. Since bootstrap 3 switched to a different box-sizing (border-box instead of content-box). So lots and lots of small ui things will fail in PF components, like icons in selectlists being wrongly positioned etc... It is very difficult to fix all these in my opinion.
If you want to use bootstrap for the responsive layout things, PF something similar to, called the grid css and more is comming