Python User In Need Of Help Setting Up And Customizing Open Source, Simple Rails App!

I would recommend looking at the forks of this inactive "project"
and picking one using a current version of Rails.

I was hoping someone could explain a couple things to me about this simple
app, such as:

Where do I change the CSS file?

Look in the directory called 'app/assets/stylesheets'.

Besides what's obvious in the readme, is there anything else I should be
aware of to make this work, or to customize, besides deployment?

Will deployment be difficult for me, someone who only knows Python, HTML,
and CSS?

Who can say? Try locally, try deploying it on your server[*] and come
back if you have specific problems.

[*] it might be easier deploying to heroku than your own server. FWIW.

Good luck.