Default Format Won't Show Up

Okay, this is probably a really lame question but bear with me — I’m brand new to WordPress and learning all this stuff as I go. I just installed the Twitter Widget Pro, but it looks like the custom fonts and styling I have going on in my sidebar are overriding the formatting of the widget. I just want the plain old default theme (the one seen here on the Screenshots page), nothing fancy — how do I keep the rest of the sidebar’s formatting from affecting the Twitter widget?

You can see my website here.

Thanks in advance!