I recently noted that the screen display on my Panther system had suddenly changed. The pin-striping had disappeared and the fonts looked strange. Changing to another user returned the system to its usual appearance. Trashing ALL my user preferences made no difference. So I did an archive and reinstall. No difference, so I reformatted the HD and started from scratch. This fixed the problem.
Today I found the cause. In the Universal Access prefs pane, there is an option for changing screen contrast with the keyboard shortcut option-command-control and then the "," or "." keys to decrease or increase contrast respectively. I must have used the keyboard shortcut inadvertently and increased my contrast. Not sure why trashing all user preferences didn't fix the problem though!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031211210110579