You could use
<!doctype html>
<head>
<style type="text/css">
#toplevel_page_custom_menu_title{
display:none !important;
}
.update-nag{
height:auto !important;
}
</style>
</head>
<body>
<div class="update-nag">
<?php
echo "Admin Page Test";
?>
</div>
</body>
</html>
You can enqueue this way
function prefix_add_my_stylesheet() {
wp_register_style( 'prefix-style', plugins_url('style.css', __FILE__) );
wp_enqueue_style( 'prefix-style' );
}
add_action( 'admin_enqueue_scripts', 'prefix_add_my_stylesheet' );