Thanks for your speedy reply! We thoroughly reviewed the ‘edit code’ section of the Newsletter plugin, but could not find a “custom CSS” section/heading as you instructed. Is the window shown under Plugins˃ Edit Plugins˃ Newsletter˃ newsletter/admin.css the correct area to add this code? There are many sub-sections listed on the right-hand side, but there isn’t one called “custom css.”
Or is it necessary that we install this plugin to add the custom CSS?
https://wordpress.org/plugins/simple-custom-css/
Sorry for any complications and thanks in advance for your help! 🙂