create a new css file with the following rule:
.bodyClass {
font-family: 'Myriad Pro', 'Lucinda Grande','Trebuchet MS', Arial, Verdana, sans-serif;
font-size: 12px;
}
then edit the HTML Editor instance.
On the Styles tab, select the new css file you have created. In the body class select list, select the new bodyClass style.