In Mac Mail Message List Font grayed oput
I am running a 27 in iMac with retina 5k display. El Capitan 10.11.4. Actually I just purchased it. I am trying to set it up so I can see things without leaning right into it and squinting (lol) or having to adjust my resolution. I have found several places inside my most used apps, but in Mac Mail, when I go to Mail>Preferences>Fonts & color, I can see "Message List Font" but it is grayed out and will not let me change it. Why is that? Can anyone help me bypass this and any suggestions on making other system things larger would be great.
Thank you,
Kathy
iPhone 5, Mac OS X (10.7.3)