Some manual setup is still required:

1. Setup Polaris styles in your layouts <head> tag:

     <link rel="stylesheet" href="https://unpkg.com/@shopify/polaris@6.6.0/dist/styles.css" />
     <%= stylesheet_link_tag 'polaris_view_components' %>

2. Define Polaris style on your <body> tag:

     <body style="<%= polaris_body_styles %>">