I am new to Mathematica. Currently I am using Mathematica 10 on Mac Yosemite.
I have tried:
Edit the
default.nbandcore.nbstylesheet. I change the font-size to 18px, and I can see it takes effect on my currently opened notebook, or any new notebooks under current Mathematica session. However, if I quit the Mathematica application (CMD+Q) and reopen Mathematica, the font-size setting of bothdefault.nbandcore.nbis reset to 13px.I open the
Option Inspector. I can search in the dialogfontand then modify theFontSize. But nothing happens, and theApplybutton is grey.Changing font-family in both
Option Inspectorand stylesheet has no effect, the current/future notebooks all display usingCourier.
I wonder what I should do to change the default font-size and font-family for all currently opened and future notebooks?

CMD+s. And if I set a different font-family (e.g.Source Code Pro, although the notebook still displays usingCourier), the new font-family remains after the reopening, but the font-size is reset to13px; therefore in some sense, I did saved thedefault.nb– Lelouch Nov 23 '15 at 11:38default1.nband then apply it successfully, without the resetting-of-font-size problem. Although change in font-family never have any effect still. – Lelouch Nov 23 '15 at 11:46