I don’t have time to do a full example just now but to get you started…
1) Call in your css file in whichever layout you are using i.e. ‘3columns’. This is located in \app\design\frontend\default\default\template\page\ Alternatively you could just add the css to boxes.css located in \skin\frontend\default\default\css\.
2) Edit view.phtml to use your style sheet. This is located in \app\design\frontend\default\default\template\catalog\product\
You’ll most likely want to call your classes on divs containing <?php echo $this->__('Product Description') ?> and <?php echo $this->__('Additional Information') ?>.