So you can add it to the Customizer > Additional CSS – and it will apply to all pages and posts.
To set it to specific pages / posts you would need to specify the unique page-id / post-id for each one in the CSS. So simpler way is to:
4. On Display Rules set the posts / pages you wish it to work on. You can use taxonomies ( categories and tags ) if you like. Or set it to the Entire Site and then exclude the ones you don’t want it on.