I would suggest to change it from transaction email
section.
If you have customised default new order email template then you can simply go to System > Transactional Email
then open the new order template then find the Payment Method
section and add inline css.
For ex:
///head part
<th align="left" width="325" bgcolor="#EAEAEA" style="font-size:13px; padding:5px 9px 6px 9px; line-height:1em;">Payment Method:</th>
//// body part
<td valign="top" style="font-size:12px; padding:7px 9px 9px 9px; border-left:1px solid #EAEAEA; border-bottom:1px solid #EAEAEA; border-right:1px solid #EAEAEA;">{{var payment_html}}</td>
If you have't customised the template then you can create one and add inline css. Learn how to add transactional email here.
OR you can simply add inline css in default template in app/locale/[your_language]/template/email/sales/order_new.html
Hope this helps.