This VQmod extension gives you a tool to add custom code right before </head> and </body> enclosing tags. For example custom stylesheets, analytics scripts etc.

It's located under System -> Settings -> Edit, at the bottom of the General tab.

Should work on OpenCart versions 2.1.0.1 and 2.1.0.2, but also on 2.0.3.1, but it's location in Settings is under Store tab.