Applying CSS styles only to certain elements

For Bootstrap 3, it’s easier if you use less:

Download the Bootstrap source code and make a style.less file like this:

.bootstrap {
    @import "/path-to-bootstrap-less.less";
    @import "/path-to-bootstrap-responsive-less.less";
}

Finally, you have to compile the less file; there are many alternatives

https://github.com/cloudhead/less.js/wiki/Command-Line-use-of-LESS
https://github.com/cloudhead/less.js/wiki/GUI-compilers-that-use-LESS.js

Or use npm to install less then compile the style.less file to style.css:

npm install -g less
lessc style.less style.css