var FILE = '/assets/css/meucss.css';
var css = document.createElement('link');
css.setAttribute('rel','stylesheet');
css.setAttribute('type', 'text/css');
css.setAttribute('href', FILE);
document.getElementsByTagName('head')[0].appendChild(css);
If you already have one link and just need to change the reference, can do so in html:
<!-- sem 'href' definido -->
<link rel='stylesheet' type='text/css' id='dinamico'/>
And in Javascript catch it by the id attribute:
var FILE = 'meucss.css';
document.getElementById('dinamico').setAttribute('href', FILE);
Furthermore, depending on how you are doing, you can use a single css file and use classes in body to define which rules should be applied in that document. For example:
/**
 * O 'body' de todas as páginas que incluirem esse css será preto.
 */
body { background-color: black }
/**
 * O 'body' das páginas que incluirem a classe 'espefico' será azul.
 */
body.especifico { background-color: skyblue }
<body class='especifico'>
  Qual a minha cor?
</body>
 
 
							
							
						 
What CSS do you want to add?
– Sergio