Usually you use CSS for styling. Use the .ui-panel .ui-panel-titlebar selector. You can find the CSS selector and all of its properties in every web browser developer toolkit. In Chrome, IE9, and Firebug, right-click the title and select "Check Item" or press "F12".
Here's an example of what it looks like in Chrome:

To get started, you can set padding to 0 .
.ui-panel .ui-panel-titlebar { padding: 0; }
Put this in the .css file that you load using <h:outputStylesheet> inside <h:body> so that it loads after CSS by default PrimeFaces.
See also:
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