If you would like to add custom CSS code to a specific page in Magento,

open that particular page via the Magento Admin Panel and, under ‘Page Information’ > click on the ‘Design’ tab. Subsequently, under ‘Page Layout’, within the ‘Update Layout XML’┬átext-box, append the following lines and save your changes.

<reference name="head">
<action method="addCss">
<stylesheet>css/your-custom-file.css</stylesheet>
</action>
</reference>

Interesting tit-bit here is that, you need not specific your theme path like, your-theme/css/.. as it’s already configured (via System > Configuration > Design > Themes in the Magento Admin Panel).Magento will generate the full URI path for your custom CSS file -> by looking up your active theme and then, a file named ‘your-custom-file.css’ (as in above example) in a sub-directory named ‘css’. The complete URI path will be like, http://yoursite.com/skin/frontend/default/your-theme/css/your-custom-file.css