I didn't find anything about this issue, but if I missed something, please point me to it.

The documentation on the ctan.org site is usually about how to use the packages in a LaTeX document, but what if I want to edit the package itself? Is there documentation available?

And on a related note: is it advisable/OK to edit the package or rather try *first* tweaking the document and only *later* edit the package? 

Editing the package certainly would affect all documents that use it in the future, so I suppose that it would be better to try changing the document options first, rather than go straight to edit the package.