Is it possible to style notifications shown by Chrome?
No, today it is not possible (January 2016).
The specification for this API doesn't include low level styling of this kind as the API should be OS and platform independent and each OS may have very different notification styling so allowing customization on a level like text color likely isn't appropriate.
It is possible that slightly more generic styling such as bold, italics, light text etc may be supported in the future and these would only apply on platforms that are able to render stylized text in notifications.