How to apply a style to a specific page?
I guess you want to apply that class to the body tag. If that's true then you just need to append body_classes array and put your class in the end.

set body_classes = [
  logged_in ? 'user-logged-in',
  not root_path ? 'path-frontpage' : 'path-' ~ root_path|clean_class,
  node_type ? 'page-node-type-' ~ node_type|clean_class,
  db_offline ? 'db-offline',
  theme.settings.navbar_position ? 'navbar-is-' ~ theme.settings.navbar_position,
  theme.has_glyphicons ? 'has-glyphicons',
  'path-product-add',
]


And then clear cache of your website.