To add an custom mail-signature in Lion you had to add an signature via preferences, and then replace the contents of the webarchive with your own. In Mountain Lion, Apple replaced the webarchive ...
Composing messages in Apple Mail: Displaying text on screen in larger size without changing font size
When composing messages in Mail.app, as I now have an HD screen on my MBP, the default Helvetica 12pt appears quite small. I would like to be able to enlarge the text size, but only locally, ...