Add a stylesheet and/or Google Font to the WordPress admin editor.

functions.php

<?php

// Register Editor Stylesheet
function text_domain_add_editor_styles() {
    // Easily add a Google Font to the Editor
    $font_url = str_replace( ',', '%2C', '//fonts.googleapis.com/css?family=Indie+Flower' );
    add_editor_style( $font_url );
    add_editor_style('editor-styles.css');
}
add_action('admin_init', 'text_domain_add_editor_styles');