For a piece of css code, use:
{source}<?php
$css = 'your css...';
$doc =& JFactory::getDocument();
$doc->addStyleDeclaration( $css );
?>{/source}
To add a css file to the head, use:
{source}<?php
$doc =& JFactory::getDocument();
$doc->addStyleSheet( JURI::root( true ).'/path/to/your/file.css' );
?>{/source}
To add multiple stylesheets/scripts, you can place 1 php block like:
{source}<?php
$doc =& JFactory::getDocument();
$css = 'your css...';
$doc->addStyleDeclaration( $css );
$doc->addStyleSheet( JURI::root( true ).'/path/to/your/file.css' );
$doc->addStyleSheet( JURI::root( true ).'/path/to/your/file2.css' );
$script = 'your script...';
$doc->addScriptDeclaration( $script );
$doc->addScript( JURI::root( true ).'/path/to/your/file.js' );
$doc->addScript( JURI::root( true ).'/path/to/your/file2.js' );
?>{/source}