Font Styles in onMessage are actually referring to the Font Weight and would be something that needs to be added via the Advanced CSS of the style applied to the text. To do this, add the following Code:
The numbers in the google font url correspond with the style's they chose in Google Fonts. For Example:

<link href="|Montserrat|Oswald:200,300,400" rel="stylesheet"> 
200 is extra light
300 is light
400 is regular

Replace the number in the font-weight CSS to reflect whichever weighting you're looking to achieve.