Add/Edit frontend theme in pure MVC based website

I am using Sitefinity 9.1 and I developed a pure MVC based website, which has all MVC widgets. 

Currently my website has just the basic frontend theme and I want to edit / add new theme to my website to make it look good. The resourcepackages folder in my web project has a Bootstrap folder but I dont see any themes, which I can edit/add.

I want a responsive website, so I want to use bootstrap themes on my website but I am totally clueless as to how to proceed. Any help is appreciated.