If you are trying to style backend (admin side) then you can use wp_enqueue_style .
check below code . Add this code in your plugin page.
define('PLUGIN_URL', plugins_url('', __FILE__ ) . "https://stackoverflow.com/");
add_action( 'admin_enqueue_scripts','my_plgin_style' );
function my_plgin_style(){
wp_enqueue_style('pluginstyle', PLUGIN_URL .'style.css');
}
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solved Styling of Custom Plugin in wordpress