CSS is cascading style sheets. Its used to layout HTML pages. You are already using it in the HTML you provided with the style attribute.
To give a background color and a small border, add the following to the div with ID twitter_div:
style="background-color: #66FF99; border: 1px solid blue"
So it reads;
<div id="twitter_div" style="background-color: #66FF99; border: 1px solid blue">
<h2 class="sidebar-title">Latest News via Twitter</h2>
<ul id="twitter_update_list"></ul>
<a href="
http://twitter.com/ferga_com" id="twitter-link" style="display:block;text-align:right;">Follow Ferguson & Associates on Twitter</a>
</div>
<script type="text/javascript" src="
http://twitter.com/javascripts/blogger.js"></script>
<script type="text/javascript" src="
http://twitter.com/statuses/user_tim...nt=5"></script>
The HEX number for the background colour is available
here, so you can change the colour to whatever you want
This may not be exactly what you are after, but its a start....