WordPress 5.0's Gutenberg editor requires enqueuing of the stylesheet as well as adding support for said editor styles before it will work properly.

add-editor-style-gutenberg.php

/**
 * Add this into your functions.php file.
 */
 
/**
 * Adding theme support
 */
if ( !function_exists( 'rwp_after_setup_theme' ) ) {
	function rwp_after_setup_theme() {
		add_theme_support( 'editor-styles' );
	}
}
add_action( 'after_setup_theme', 'rwp_after_setup_theme' );

/**
 * Adding editor stylesheet
 * Replace {stylesheet_uri} with the URI to your editor/Gutenberg stylesheet.
 */
if ( !function_exists( 'rwp_admin_init' ) ) {
	function rwp_admin_init() {
		add_editor_style( get_theme_file_uri( '{stylesheet_uri}' ) );
	}
}
add_action( 'admin_init', 'rwp_admin_init' );