It depends on the Xml package you're using to display the product. I'm going to use product.SimpleProduct.xml.config as an example.
In most Xml packages it's going to be called "Description" or the variable "$pDescription" (as is the case in product.SimpleProduct.xml.config). Look for that in the Xml package, and in most cases you'll find it's in a div tag. Unfortunately, the div tag has no class on it to attach a style, so you'd need ot do that. Like so:
<div><xsl:value-of select="$pDescription" disable-output-escaping="yes"/></div>
<div class="descriptionProductDiv"><xsl:value-of select="$pDescription" disable-output-escaping="yes"/></div>
Then you could add your style to style.css in App_Themes\Skin_1.