application.rb

class Application < Sinatra::Base
  configure do
    enable :static
    set :views, File.join(File.dirname(__FILE__), 'views')
  end

  get '/' do
    slim :index
  end

  get '/status', :provides => :json do
    color = rand(2).even? ? 'red' : 'green'
    {color: color}.to_json
  end
end

index.slim

h1 Hello World!
javascript:
  $(document).ready(function() {
    $("#foo").click(function() {
      $.get('/status', function(data) {
        $("#bar").attr("style", "color: " + data.color);
      });
    });
  });
button type="button" id="foo" Update
.status#bar style="color: black" foo bar baz

layout.slim

doctype 5
html xmlns='http://www.w3.org/1999/xhtml'
  head
    title Application
    link rel='stylesheet' type='text/css' href='/css/style.css'
    meta content='text/html; charset=utf-8' http-equiv='Content-Type'
    script src="//ajax.googleapis.com/ajax/libs/jquery/2.0.0/jquery.min.js"
  body
    == yield