blue_yellow.haml

!!!
%html
  %head
    %style(type="text/css")
      :plain
        body { 
          color: blue; 
          background: yellow;
        }
  %body
    %h1 Blue Yellow Template
    = content

config.ru

require 'rubygems'
require 'haml'
require 'tilt'
require 'layout'

app = lambda {|env| [200, {'Content-Type' => 'text/html', 'X-Layout' => 'blue_yellow'}, 'Hello World'] }
use Layout
run app

layout.rb

class Layout
  def initialize(app)
    @app = app
  end
  
  def call(env)
    status, headers, response = @app.call(env)
    new_response = layout(build_response_body(response), headers)
    new_headers = recalculate_body_length(headers, new_response) 
    [status, new_headers, new_response]
  end

  def layout(response, headers)
    template = Tilt.new(headers['X-Layout'] + ".haml")
    template.render(nil, :content => response)
  end

  # Borrowed from Rack::Honeypot
  def build_response_body(response)
    response_body = ""
    response.each { |part| response_body += part }
    response_body
  end
  
  def recalculate_body_length(headers, body)
    new_headers = headers
    new_headers["Content-Length"] = body.length.to_s
    new_headers
  end
end

red_black.haml

!!!
%html
  %head
    %style(type="text/css")
      :plain
        body { 
          color: red; 
          background: black;
        }
  %body
    %h1 Red Black Template
    = content