mastodon/app/views/layouts/error.html.haml

!!!
%html{ lang: I18n.locale }
  %head
    %meta{ content: 'text/html; charset=UTF-8', 'http-equiv' => 'Content-Type' }/
    %meta{ charset: 'utf-8' }/
    %title= yield :page_title
    %meta{ content: 'width=device-width,initial-scale=1', name: 'viewport' }/
    %link{ href: 'https://fonts.googleapis.com/css?family=Roboto:400', rel: 'stylesheet' }/
    :css
      body {
        font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
        background: #282c37;
        color: #9baec8;
        text-align: center;
        margin: 0;
        padding: 20px;
      }

      .dialog img {
        display: block;
        margin: 20px auto;
        margin-top: 50px;
        max-width: 600px;
        width: 100%;
        height: auto;
      }

      .dialog h1 {
        font: 20px/28px -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
        font-weight: 400;
      }
  %body
    .dialog
      %img{ alt: 'Mastodon', src: '/oops.png' }/
      %div
        %h1= yield :content