WordPress theme editor is missing

It is probably because of iThemes Security and its settings. So yeah, just go to wp-config and change DISALLOW_FILE_EDIT to false.

define( 'DISALLOW_FILE_EDIT', false );

Or just switch it off here: Dashboard -> Security -> WordPress Tweaks -> Configure Settings

Uncheck: Disable File Editor

Open up your wp-config.php file, and search for

define('DISALLOW_FILE_EDIT', true);

Change true to false:

define('DISALLOW_FILE_EDIT', false);

For those who are using Siteground hosting define('DISALLOW_FILE_EDIT', false); won't work. Sitegound hosting provide SG Security plugin by default on direct WordPress installation from cPanel and it blocks the Theme Editor and Plugin Editor option by default.

To enable the Theme Editor and Plugin Editor option you just need to disable the option called Disable Themes & Plugins Editor under Site Security. Here is the path to disable that option :

Click on SG Security -> Site Security -> Disable Themes & Plugins Editor Option

That's it. I hope you find it useful.