This article is from the Macintosh application software FAQ, by Elliotte Harold firstname.lastname@example.org with numerous contributions by others.
Make a copy of TeachText 7.0 and open the COPY with ResEdit.
Open CODE resource 1. You'll probably be warned that the resource is
stored compressed and that opening it will irreversibly decompress
it. Click OK. Scroll down to address 4A88. You should see the hex
string "0001 A887". A887 is the call to TextFont(). The four hex
digits preceding it (0001) are the font ID. Change this number to
the ID (in hexadecimal) of the font you want. Monaco would be
0004. (It may be something else if Monaco has been renumbered
on your system.)
To change the size go to the next line (4A90) and look for
"000C A88A" A88A is the call to TextSize(). The four hex digits
preceding it are the size of the font to be used. Change "000C" to
the size (in hex) you want. For instance 0009 is nine-point, 0010
would be sixteen point.
Changing the font and size can adversely affect the way
TeachText displays embedded pictures which most commonly occur in
read-only TeachText documents (the ones with the little newspaper
icons) so you may want to finish your modifications by deleting FREF
resource 130 to prevent your modified TeachText from opening those
files. Save your changes and quit.