To answer the question, from the codex, listed above in the comments.
function load_custom_wp_admin_style($hook) {
// Load only on ?page=mypluginname
if($hook != 'toplevel_page_mypluginname') {
return;
}
wp_enqueue_style( 'custom_wp_admin_css', plugins_url('admin-style.css', __FILE__) );
}
add_action( 'admin_enqueue_scripts', 'load_custom_wp_admin_style' );
Note: If you are unsure what your $hook name is .. use this to determine your hookname. Put the code after the { from the function.
wp_die($hook);