If you wanted to change the font, size, or colour of your messages in Gmail, you normally have to do so on a mail-by-mail basis. Not so anymore, if you enable a new default text styling option from the Labs.
After enabling the new feature in the Labs menu from Gmail's settings, head back to the "General" tab and look for the new text box. Set your font, size, colour, and other options there, and they'll stick from message to message. Most of us probably don't need 18-point purple Garamond text, but, then again, a few subtle changes might help your own missives stand out in your message view.
New in Labs: Default text styling [Official Gmail Blog]