To learn chords, you don't really need theory, just memory and practice.
To learn about chords, start with basic music/chord theory.
Theory explanations are all over the internet. Good luck!
Edit: But then with music theory, you get so much more than just the
reason some chords have 7's in them and so on, you learn why they
are built the way they are, how they relate to eachother and much more.
So if you have the interest and patience, study study study!