using FireFox with the firebug plugin, you can inspect the power store pages to find the specific elements you want to apply the font to to find the class name or ID for the div. For example, the long description div has an id of "ProductLongDesc":
<div id="ProductLongDesc">
here is an article that talks about using Firebug:
roadmap_10.php
once you know the div class name of ID, you can create a css rule to apply the calibri font. you will probably want to use a font-family, this way you can set alternate fonts that will be used if the calibri font is not installed. you would want to add your custom styles to the webassist/themes/<your theme>/css/configurable.css file at the end, for example:
#ProductLongDesc {
font-family: Calibri, Candara, Segoe, "Segoe UI", Optima, Arial, sans-serif;
}