Styling Custom Fields

I’d like to add a background color to my custom fields, but when I apply the style, the background color goes across the entire screen. I’m using the_meta tag for my custom fields which returns:

<ul class='post-meta'>
<li><span class='post-meta-key'>Project:</span> Prozac</li>
<li><span class='post-meta-key'>URL:</span> www.www.com</li>

</ul>

As you can see, there’s only a span around the ‘key’ but not around the value. How do I apply a background color only around the text? I’d also like to add padding to the bottom of each custom field.