Add individual styles to some pages.

To use main.tpl for adding some styles wich are for all pages - i know that.

But if i want to add some individual styles how I can do it?
One man recommended to put string with CSS to page-file, at example to viewtopic.php.

echo('<link rel="stylesheet" type="text/css" media="screen" href="http://htmlmaster.ru/forum/style/Oxygen/level2.css" />');

Wherever I put it, during watching html-source, this string is in top of page over Doctype. And even if there wouldn't this trouble, I don't understand where put it correctly: such a hard php-code, wich I don't understand (I'm front-end developer).

So, I need some help.