Theme editor disabled

I need to use the theme editor to edit header.php but I cannot find the editor anymore. I need to insert a different short code into header.php for each subsite in multisite.

Searching on Google, I found "Enabling the Theme and Plugin Editors in wp-config.php" at https://community.1and1.com/enabling-the-wordpress-theme-and-plugin-editor-wordpress-free-mode/

In wp-config.php, I located:
define( 'DISALLOW_FILE_EDIT', true ); //disables the plugin and theme editor - http://premium.wpmudev.org/blog/keeping-wordpress-secure-the-ultimate-guide/

I changed this to:
define( 'DISALLOW_FILE_EDIT', false ); //disables the plugin and theme editor - http://premium.wpmudev.org/blog/keeping-wordpress-secure-the-ultimate-guide/

However, I still do not see the editor. Can someone tell me what to do? I could