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

!!!
%html{:lang => "en"}
  %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