Applies defined CSS classes to the ACF WYSIWYG editor so that you can individually style them.

custom-acf-wysiwyg-css.php

<?php
function hwid_acf_admin_footer() {
	?>
	<script>
		( function( $) {
			acf.add_filter( 'wysiwyg_tinymce_settings', function( mceInit, id ) {
				// grab the classes defined within the field admin and put them in an array

				var classes = $( '#' + id ).closest( '.acf-field-wysiwyg' ).attr( 'class' );

				if ( classes === undefined ) {
					return mceInit;
				}

				var classArr = classes.split( ' ' ),
					newClasses = '';

				// step through the applied classes and only use those that start with the 'hwid-' prefix
				for ( var i=0; i<classArr.length; i++ ) {
					if ( classArr[i].indexOf( 'hwid-' ) === 0 ) {
						newClasses += ' ' + classArr[i];
					}
				}
				// apply the prefixed classes to the body_class property, which will then
				// put those classes on the rendered iframe's body tag
				mceInit.body_class += newClasses;
				return mceInit;
			});
		})( jQuery );
	</script>
<?php
}

add_action('acf/input/admin_footer', 'hwid_acf_admin_footer');

editor-styles.css

.hwid-field-group {
	font-family: Arial, "Helvetica Neue", Helvetica, sans-serif !important;
}

.hwid-field-group.hwid-dark {
	background: #000;
	color: #fff;
}

.hwid-field-group.hwid-light {
	background: #eee;
	color: #333;
}

.hwid-field-group.hwid-red {
	background: #fff;
	color: #f00;
}

functions.php

<?php
function hwid_theme_add_editor_styles() {
	add_editor_style( 'css/editor-style.css' );
}
add_action( 'admin_init', 'hwid_theme_add_editor_styles' );

Your last change, line 20 of the prefix 'hwid-' to 'rmg-' makes no sense and breaks the code.
I've changed it in the version I have used and it works.
Nice piece of work.