In what program(s) are the front-ends of Treehouse and other similar-looking sites designed?

Yes, treehouse is a Ruby on Rails site.

For front-end, it's HTML5, CSS3, and Javascript (or JQuery library) that all tie together to create a nice front end experience. All of the nice, big sites are going to be using those three languages on the front end.

It's not so much the languages used that makes the difference between a site like teamtreehouse and Smells Like Bakin' but it's much more how the language is used combined with a solid understanding of design principals, color theory, etc.

Frontend markup/languages include HTML5(HTML5, CSS/CSS3, JavaScript). It is also very likely that some of those sites are using jQuery as a JavaScript Framework. Today, it is also possible that some modern designed sites may be using a CSS framework like Twitter Bootstrap or Zurb Foundation, at the very least for their CSS scaffolding and layout. What makes these sites you mentioned above look 'good' is the skill of the frontend designer(s) and User Interface engineer(s). Much of what you actually see is probably creative uses of CSS and JavaScript, which work together along side HTML to create the page layouts and functionality. The above mentioned languages are front end languages, but the others you mentioned like PHP, Ruby/Ruby on Rails, Python are backend/server side languages. While they do not contribute to the site's design at all, (if used) they contribute to the dynamic content of the site. So a language like PHP would be used to output information, but PHP itself does not control the sites design unless it's output is another front-end language (HTML, CSS, JS) etc. PHP (or other backend languages) can effect what you see on the page, but probably only because the backend language is being used to generate front-end content.