In the Quintus theme, the widget title styles are set using this CSS:
.widget .widget-title {
border-bottom: 1px solid #e7d9b9;
color: #554b34;
display: inline-block;
font-size: 14px;
font-weight: 400;
padding: 4px;
}
There's also rule a little higher up that sets the font:
.widget .widget-title, .blog-footer, .entry-content label {
font-family: Lato, sans-serif;
}
Source: https://s-ssl.wordpress.com/wp-content/themes/pub/quintus/style.css?m=1315816683g&minify=false
Using the "#secondary h1" selector that devblog mentioned will also work to adjust those titles.
If you ever want to find out which selector an area is using, you can use the right-click → Inspect Element option in Chrome or Safari and it will show you the HTML and CSS being used for whatever you clicked on. The Firebug add-on for Firefox also has the right-click → Inspect Element option. It's pretty nifty if you want to check it out!