add css style to template tag

Hi,
I would like to add css styling to the post title and post date template tags. I have added the following to post-header:

<h1>
	{{post_title}}
</h1>

<div class="title">
	<span class="post-title">{{post_author}}</span>
	<span class="post-date">{{post_date}}</span>
 </div>

And the following to simpleCSS:

.post-title {
 	font-size: 18px;
}

.post-date {
   	font-size: 9px;
}

However, the styling doesn't appear to have any effect in the browser. Could you help please?