can someone tell me how do i download a bootstrap theme?
for example i want to save this one
http://bootswatch.com/cerulean/

There's a download button at the top, which reveals a sub menu with the CSS files (a bootstrap.css for development and bootstrap.min.css for production). If you're familliar with CSS preprocessors then you can choose the others in the submenu, but I assume you don't use a preprocessor, so you need the first two.

Then replace the default css files that comes with the bootstrap download (http://getbootstrap.com/) with these ones.