In a previous video, we learned how to use DiffMerge as a git difftool and git mergetool. As I was creating that video, I noticed a bug in the program that crashed DiffMerge when attempting to change the font-size. This bug has not yet been fixed by DiffMerge; however, there is a workaround.
All we need to do in open the preferences from the terminal and change the font-size manually.