WordPress.org

Why don't you support authentication?

Because the processing is done in Javascript, not PHP, so your password would be clearly visible.

Then why is this in Javascript and not PHP?

Twitter imposes a 100 request per hour limit on single users or IPs. If this was written in PHP, all the requests would come from one single source (your web host). In this case, once you exceeded 100 views in an hour, visitors to your blog would no longer receive any content for the tweetrix plugin. By writing the functionality in javascript, the requests are coming from the client rather than your server.

How do I change the style?

In your theme css file (or somewhere on your page), add styles for #tweetrix (if you've left the div id option set to tweetrix). The individual words are wrapped in span tags.