How to add css for custom pages

<?php if ( is_page(array('somepage1', 'somepage2', 'somepage3'))) {
echo ' class="myclass" '; } ?>


or

<?php 
if ( is_page (blog)){ 
?>

<style>
.wrapper
{
background:black !important;
}
</style>

<?php
}
?>

This is custom css for blog page only.