Your Theme editor is hidden by default because the iTheme Security plugin is active.

There is an option to turn off the Theme Editor in the iTheme Security. Please read this article for more details – http://www.jamesandchey.net/plugins-ithemes-security-how-to-enabledisable-the-file-editor-to-edit-wordpress-php-files/.

If that is not the case, please check your wp-config.php file and make sure that it does not contain the following code:​

define( 'DISALLOW_FILE_EDIT', true );
define( 'DISALLOW_FILE_MODS', true );

If it does, delete it or change the value from “TRUE” to “FALSE”.