Changing default quit key for apps on macos

The following command will change the default key to Command+Shift+Q for Mail and Safari.

defaults write NSGlobalDomain NSUserKeyEquivalents '{"Quit Safari" = "@$Q"; "Quit Mail" = "@$Q";}'

The odd characters before the 'Q' in the previous command, specify the modifiers:

• @ = Command 
• $ = Shift 
• ~ = Option 
• ^ = Control