It’s really simple. In Appearance > Menus, look at the top right of the screen. Click on Screen Options Tick the box for Link in new tab That’s it! Now you will see that option when editing the custom link in the menu. Thanks to this great post at RichWP for the tip.
Just came across this great article from Max Foundry (author of my favourite buttons plugin, MaxButtons). The article describes really clearly how the order of precedence works with CSS. Precedence in CSS (When Order of CSS Matters)