I tried that, funny thing is, I can change the font family, but I can't change font size.
Well, that's better than nothing, as I can change it to another font that looks larger even at the same 11pt.

I tried modifying (1) main.css, (2) Info.plist (in the package) to change the font, but to no avail... would you have any other suggestions?

Can you let us know what file in the package to edit in order to change the message font?
I find that the default (I'm assuming Arial 11?) is too small; I usually use Helvetica 12.