In order to enable this feature without the Debug menu all you must do is add the
CalUIUseSystemHighlightColorForToday key to the ~/Library/Preferences/com.apple.iCal.plist preferences file using any plain text editor (both TextEdit and TextWrangler would work).
First quit Calendar, then edit the plist, and upon relaunching Calendar it should be using the system highlight color selected in the General» Appearance panel in System Preferences.
[crarko adds: I tried this with the 10.8.2 Calendar.app and I believe it worked. I opened the plist in BBEdit and copied and pasted an existing key/value combination, edited the key to match the name above, and set the value to
true. I saw some highlighting respect this, although I can't say I'm completely sure what all the intended effects are. Perhaps the submitter will see this and clarify what to look for.]

