Most of this is stock HTML and CSS from the Readme.io website.
For the most part, the only changes you will need to make are in Phoenix_files/custom.scss.
Make sure to install Sass and then run
sass --watch ./Phoenix_files/custom.scss
That will make sure to update the css as you are working on it.
Before copying custom.css to Readme.io, remember to remove the selector for
.header h1.navbar-brand a. That is only used in development so the logo shows up.