The easiest way to style your editor is by applying your theme’s stylesheet, making content look the same while editing as it does in your content.
Put the following in your functions.php file:
add_action('init', function() {
add_editor_style(get_template_directory_uri().'/style.css');
});
You may need to adjust the path on line 2; the above assumes that style.css is in the root of your theme and contains theme styles. I personally prefer this file to contain just the WordPress stylesheet header and for the actual theme styles to be located in /css/style.css, but that’s a matter of preference.