A simple php function to add a meta box (add_meta_box) to posts/pages and write it on the footer (wp_footer).

add_meta_box.php

<?php

/******
Simple function to add custom content to posts/pages on WordPress.
Just add to functions.php
******/

function custom_css_add() {
   $post_id = $_GET['post'] ? $_GET['post'] : $_POST['post_ID'] ;
   add_meta_box( 
      'custom-css', 
      'Custom CSS', 
      'custom_css', 
      'page', // post type
      'normal', // options: side
      'low' ); // options: default, high
}

function custom_css_save( $post_ID ) {
   if( isset($_POST) ) {
      update_post_meta( $post_ID, 'custom-css', $_POST['custom-css'] );
   }
}
add_action( 'add_meta_boxes', 'custom_css_add' );
add_action( 'save_post', 'custom_css_save' );

function custom_css( $post ) { 
   $cwid = get_post_meta( $post->ID, 'custom-css', true ); ?>
   <p>
      <textarea name='custom-css' id='custom-css' cols="120" rows="10"><?php if( $cwid ) { echo $cwid; } ?></textarea>
   </p><?php
}

function custom_css_write() { ?>
   <style>
      <?php echo get_post_meta( get_the_ID(), 'custom-css', true ); ?>
   </style><?php
}
add_action( 'wp_footer', 'custom_css_write', 100 );