Either Edit app\design\frontend\YourInt\YourTheme\layout\page.xml
add the following line to the head block
* ensure your theme is selected In under custom design (admin panel -either cms/manage pages or manage categories/category depending on how you created your page.
To include a different CSS page which is not global, you can also add the following code into your “custom Design” / “Layout Update XML”
section of the page you wish to use an alternate CSS