If you can’t use your beloved Monaco font in AppCode because it has gained weight and simply looks too bold, you might want to trim your anti-aliasing settings.
Try running this in your terminal
$ defaults write com.jetbrains.AppCode.plist AppleFontSmoothing -int 1
Left one is updated and right one is original
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You might tweak it even more by using different values for -int 1 e.g. 0, 2 or 3. I’m not changing the system wide font smoothing setting, but only the one for AppCode.