veinjs is a library that injects CSS into the document (it's a perfectly legal procedure). Rather than using inline styles, the CSS is being pushed into a dynamically generated stylesheet that's injected into the header.
This approach gives us a couple of benefits:
Persistence - Say you want to dynamically make all your sub headers a certain color. You apply CSS properties with jQuery, and everything looks great. But then, you load some more stuff with AJAX, and you notice that your CSS is useless, because it has been only applied to the elements currently living in DOM. That's when injection can save you headache. Just apply any CSS code once, and it's valid - until the next refresh!
Performance - Our minions at jsperf.com tell us that injecting CSS rules for a vast number of elements is VERY fast, check it out for yourself: http://jsperf.com/inject-vs-apply/2
Media Query Support - You can inject media queries and work responsive magic with veinjs dynamically.