I really like the SF Mono font Apple includes in Sierra and High Sierra (even more than Menlo), but for some reason it’s only available inside of Terminal and Xcode. So I’m out of luck when I’m writing in MarsEdit, BBEdit, or any other third party app. Fortunately, you can just copy the font from Terminal’s app bundle and install it so it’s available everywhere.
Run this command from the Terminal and you’re all set:
cp -R /Applications/Utilities/Terminal.app/Contents/Resources/Fonts/. /Library/Fonts/
You could also ⌘-G in Finder and copy the files over that way if you prefer.