I am looking to build a myth box but wanted to make a few changes to the code. First what language is myth developed in. Is it php? Also anyone know of any good tutorials about theme customization for myth that are out there.
Depends on what part of myth you're looking at. The main codebase is C/C++. The themes are a combination of png images with xml scripting. I don't have the other parts of myth sourcecode downloaded, so no idea.
And mythweb has a ton of code in php to drive the web engine. There are contribed scripts in all sorts of languages (but perl shows up a lot). But that is pretty much it on the language front. I guess it should be mentioned that mythtv uses QT as it's windowing library. It is a technology to know all to itself.
Give us an idea of what you want to customize (or ask the same question on myth-dev AT mythtv DOT org). You will get a more specific answer.