change MacOS font on notifications

I am looking to change the osx font on notifications. I find the font used on the notification bubbles on the right of the screen too grey and difficult to read. I am looking to make the font a lot "blacker" if possible. This is shown below.

Interestingly, I find the font apple uses for the same notifications in notification center much better. These notifications have bold black headers, which look nicer, although the grey text remains for the remainder.

Can the font be changed to be blacker in the notification bubbles?