The installation of this extension is very simple. Just connect to the Typo3 Extension Repository and download «Page StyleSheet Selector» (css_select), located in the backend section.

You can there specify the folder where you will store your CSS files (you must manually create this folder). By default, it's fileadmin/css/.

You can also choose here the allowed extensions for the CSS files.

Once the extension is installed, you can specify one or more stylesheet(s) for each of your pages. This is done through the page headers:

You'll find in the selector all the CSS files located in the directory you've set during the install process. Just click on a filename to add it.