For centering: http://masonry.desandro.com/options.html#isfitwidth
As for the responsiveness, you’ll want to look into media queries. I would certainly end up with a single stylesheet that you’re loading to your users but most professional front-end developers use some sort of pre-processor like LESS or Sass which means we’re able to set up lots of smaller files that all get built into one file on save.
So you might have something like:
/styles _header.scss _layout.scss _footer.scss _typography.scss _mixins.scss style.scss
style.scss includes all of the other stylesheets into it. http://sass-lang.com/guide