use strict in adminimal_theme.js

Adding 'use strict' to adminimal_theme.js has two major benefits.

The first is maintainability, as it turns common mistakes - which are silently ignored in regular Javascript - into errors.

The second is performance, since browser engines can optimize the script better, as they know they don't have to work around possible allowable mistakes.

The script is already 'use strict' compliant, so this would be an easy win.

Here's is the the MDN documentation for those who are unfamiliar or want to learn more.