You want to serve different css by detecting the screen size.
This tool does it: Adapt.js (Take a look at Less too)
With Adapt.js you can do stuff like this:
// Edit to suit your needs.
var ADAPT_CONFIG = {
// Where is your CSS?
path: 'assets/css/',
// false = Only run once, when page first loads.
// true = Change on window resize and page tilt.
dynamic: true,
// Optional callback... myCallback(i, width)
callback: myCallback,
// First range entry is the minimum.
// Last range entry is the maximum.
// Separate ranges by "to" keyword.
range: [
'0px to 760px = mobile.css',
'760px to 980px = 720.css',
'980px to 1280px = 960.css',
'1280px to 1600px = 1200.css',
'1600px to 1920px = 1560.css',
'1940px to 2540px = 1920.css',
'2540px = 2520.css'
]
};