theme.xml is just settings, nothing more
otherwise good old php, html, css and javasript.
I am not aware if there is any official documentation, but you can look at e107.org, maybe you find something usefull.
You can look at my old (OLD is important word, some things work different way now) to get picture:
But it's easy to look at any new theme (or updated Solid theme) to see what is done new way. And there is Starter theme I use for my themes, if you want.
Oh, and it's good practice if somebody answers your questions (you have some other already here) to add some feedback there. To know if your questions can be closed.