CSS widget title

please use a tool such as Firebug http://getfirebug.com/ to investigate the involved style selectors.

there are other ways to find out:

you can look into the html code of your site in the browser (browser toolbar ‘view’ or ‘view source’) and try to locate the rendered html of the sidebar widget;

many browsers also have something like ‘inspect element’ if you right click on some element on your site;

or you could look into functions.php of your theme where the sidebar is registered: http://codex.wordpress.org/Function_Reference/register_sidebar