Hm, weird question. It is bootstrap, so in bootstrap file? Open source code of page and you will see. You can override them in custom_style.css (or style_custom.css - I still don't know what is correct out of head)
I hope you create copy of core bootstrap3 theme because if you use style.css, then you lost your changes.
And maybe you can look at other bootstrap3 themes to see how they do this - f.e. agency?
Or bootstrap3 has bootstwatch switcher, so again, look how it is used in theme.php