Where can I find the theory about compound sentences?
Hmm, an advanced subject! There are many resources in Greek about. The most comprehensive and practical I found is here http://users.sch.gr/ipap/Ellinikos%20Politismos/Yliko/Theoria%20Nea/Protasi,%20Eidi%20protaseon.htm
A blog with theory and practice of sentences here: https://e-didaskalia.blogspot.gr/2014/06/blog-post_1349.html
A Google Translation could give you an idea. The examples are not difficult to understand.
A webpage in English with a lot of examples, but not complete is also this one: http://foundalis.com/lan/grkmore1.htm (this site is a pure diamond for the English speaker who tries to learn Greek)