Safari 6With the release of Safari 6, the default font settings preferences have been removed. If you still want to set default fonts without using a custom style sheet you can still use Terminal commands to set them. Another workaround is to use the Quickstyle Safari Extension.

Below are some example commands for Terminal:

Proportional Font:

defaults write 'Lucida Grande'
defaults write 16

Fixed Width Font:

defaults write Monaco
defaults write 12