How Can I change Font in FileMerge

I am using High Sierra 10.13.6, with Xcode 9.4.1, FileMerge 2.11.

I want to change the font size in FileMerge, the default is Menlo 11.0, which is too small for me. So I go to preference-Options-set, and choose a larger size, but it doesn't work at all.

I tried google to find the answer, xcode - How to change FileMerge's font/settings/preferences? - Stack Overflow

but I can't find this xccolortheme file. All the xccolortheme files I found are for xcode, but not for FileMerge. In fact, I didn't even find the folder

~/Library/Developer/FileMerge

Can someone help me to solve the problem?