Enqueue Admin Styles

wds_enqueue_admin_styles.php

<?php
function my_plugin_enqueue($hook) {
    if ( 'edit.php' === $hook ) {
        $file_path = plugin_dir_path( __FILE__ ) . 'css/edit_page_styles.css';
        $version = filemtime( $file_path );
        wp_register_style( 
            'my_plugin_main_css', 
            plugins_url( '/css/edit_page_styles.css', __FILE__ ), 
            array(), 
            $version
        );
        wp_enqueue_style( 'my_plugin_main_css' );
    }
}

add_action( 'admin_enqueue_scripts', 'my_plugin_enqueue' );