CSS Customizations for certain page

If you have a lot of rules that you need to conditionally apply (there’s no hard and fast rule, but, to make one up, maybe 10-15+ lines?) then @aahan’s answer is the way to go. If you only have a few though, just work off body class.

In your header.php file (that’s the common name at least) make sure the <body> has:

<body <?php body_class(); ?>>

Then you’ll get lots of classes you can use to write conditional styles like

.page-123 .entry-content
.template-my-custom-template-php h1

etc…