There are two separate options for changing the source fonts, one for files opening with a SBCS/DBCS encoding, and another for files opening with a Unicode encoding. I generally set both to the same font.
Go to the "Tools -> Options" dialog and then go to Appearance > Fonts. Under the list labelled "Element", "Unicode Source Windows" and "SBCS/DBCS Source Windows" are the two font types you'll need to set.