Hi,
In Messages > Preferences > Viewing are the settings for your own Outgoing Fonts and colours plus the one to override incoming ones.
To change your own you have to change the Background colour first (The Balloon colour by default)
You can then change the Font and the Default is Helvetica.
You can also change the colour of the text if your wish.
The Senders options work the same way.
The default font is Helvetica if no Font is sent in the balloon info.
Leaving it at Automatic means the colours and Fonts of those that can alter their choices then show up as such in the conversation/chat.
The iOS does not send colour in the data that is sent and if you don't have the font the iOS is using then it should be Helvetica that you see.
You can therefore change the Fonts to the choice to be shown.
It will mean those iChat and Messages buddies that use Colour and Fonts will only show as Helvetica.

10:18 pm Saturday; November 2, 2013
iMac 2.5Ghz 5i 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
Message was edited by: Ralph Johns (UK)
Message was edited by: Ralph Johns (UK)