change font-family in message template body using javascript

Hi Everyone,

I wrote a Javascript to fetch the template and populate on the message body.  Now i would like to change the font-family and font-size of the content in the message body. 

i tried  this following code inside the javascript and its not working. 

document.body.style.fontSize="10px";
document.body.style.fontFamily="somefont"

or

* {
font-family: somefont ;
font-size: 10px;
}                                      

any other suggestions are welcome.