Date based CSS/theme

I php you can create an anchor class based on the desired date.
I haven’t tested it, but it should be something like this:

    // your relevant html parent element
    <div class="<?php echo getOlderPostsClassName( '5/5/2012',the_time() ); ?>">

    function getOlderPostsClassName($untilDate, $wordpressPostDate)
    {
       $start = strtotime($untilDate);
       $end = strtotime($wordpressPostDate);
       if ($start-$end > 0)
       {
          return 'My_css_class_for_older_posts';
       }
       else
       {
       return '';
       }
    }

Then in your Css you can target differently any descendant element:

.My_css_class_for_older_posts h2{
    color:red;
 }