2
They are in your style.css in your theme directory.
To find out what classes are being used on what, I recommend using Mozilla Firefox as your browser with the WebDeveloper extension available from here:
http://www.chrispederick.com/work/firefox/webdeveloper/Install. Click Information. Choose Display Class & ID Information.
I don't know diddly squat about css, but this little plugin told me what to change to make my site look juuuuuuust right.