Snippet from my book Mac Kung Fu ($1.99 at Amazon):

Change Fonts Used in Finder

Here is a personalization tip with a little productivity usefulness thrown in. Like all Mac apps, the Finder uses the Lucida Grande font. However, you can change both the title bar font and the font elsewhere in the program window, such as the font used for displaying file and folder names. Doing so can improve at-a-glance viewing of Finder windows. Changing the Main Finder Font To change the fonts used throughout Finder (except for Icon view), open a Terminal window (open Finder, select the Applications list, and then in the list of applications double-click Terminal within the Utilities folder), and type the following:

defaults write com.apple.finder NSSystemFont -string AmericanTypewriter;killall Finder

I’ve specified the American Typewriter font, but the key is to type the whole name of the font as it is listed in applications like TextEdit, but remove any spaces between the words in the font name. So, the American Typewriter font becomes AmericanTypewriter.

Changing the Title Bar’s Font

To specifically change the font of a title bar, type the following into a Terminal window:

defaults write com.apple.finder NSTitleBarFont -string ArialBlack;killall Finder

Here I’ve specified the Arial Black font, again removing the spaces between the words in the font title.

You might also want to increase the size of the title bar font. Use the following Terminal command to increase the point size to 16:

defaults write com.apple.finder NSTitleBarFontSize 16;killall Finder

Changing the Fonts for Other Apps

This tip can be used to change the fonts of other apps in a similar way, although not always with a high rate of success. To give it a try with an app, first quit the app in question, and then type the following into a Terminal window, typing the app’s name after the -app component (type it exactly as it appears in the Applications list of Finder); the following will set the American Typewriter font to be used in the title bars of TextEdit windows:

defaults write -app TextEdit NSTitleBarFont -string AmericanTypewriter

The following line will set the title bar font for the Notes app to the Stencil font:

defaults write -app Notes NSTitleBarFont -string Stencil

To revert to the default system font, modify the following line, again typing the app’s name after the -app component; the following reverts TextEdit:

defaults delete -app TextEdit NSTitleBarFont