app.py
static \
    style.css
    reset.css
templates \
    index.html

(r"/static/(.*)", tornado.web.StaticFileHandler, {"path": "static/"})

<link rel="stylesheet" href="/static/style.css">

import os
application = tornado.web.Application(
        static_path=os.path.join(os.path.dirname(__file__), "static"),
        template_path=os.path.join(os.path.dirname(__file__), "templates"),
    )