Every modification to PB (with very few exceptions) is written with a combination of HTML, CSS, and JavaScript. Only the extremely advanced ones incorporate PHP or another server side language.

If you're looking to learn, start with learning HTML. you should be able to read ProBoard's HTML source before trying to modify it with JavaScript (you need to know what you're modifying, right?) After that, I would think it's worthwhile to learn CSS, but you don't necessarily have to before jumping into JavaScript.