Where is the html file?

I’m trying to edit the font size in my sidebar widgets. A “view source” of my page shows div classes named text_widget, etc. but there is no mention of such classes in the style.css or sidebar code and there is no other .css file. If I highlight the sidebar with Firebug it also shows the widget classes as being in an html file. I assume WordPress assembles this page by calling all the parts but from editor I cannot find the widget text section in any of the theme content files.

Where is the html file to edit or how else do I edit widget type size?

The page in question is at http://jonsanchez.net/welcome/