Nice plugin. Have seen wonderful results on other sites. However, duplicating those results (ie, styling via CSS) is not for the faint of heart. After 3 hours of searching through support messages and a glass of The Macallan, I may just try to convince the client to use the "official" ugly twitter plugin.
I can understand not including a CSS file for reasons of "everyone wants different styling", but including one would at least help people determine what to EDIT. I understand the benevolence of a free widget, but when it's this difficult to add to a site, one feels more annoyed than gratified.
If you're an expert with function.php, you may have no problems. If you're simply a designer, prepare for battle. Sigh.