How to improve from ~1200? I am not new to the game of Chess. I have been (casually) playing it for over a decade, and I would like to get better at it. 

I know the basic rules of the game very well, I understand basic tactics (forks, pins, skewers, zugzwang), good bishops vs. bad bishops, center advantage, etc, and I believe my core principles are pretty good. 

However, I do not have any actual theory memorized, and I believe I miss out on some positions. 

I would like to understand the game better and get to 1500 by the end of the year. I am willing to put in time and effort. What resources/books would you recommend?
@Arka Mukherjee, I find most players under 2100 weak tactically, so at 1200, you are certainly weaker than you understand you are. I do not say that to be mean, but just as a very logical person, who has been a reasonably strong player for over 30 years.

At your level, memorizing theory is mostly a waste of time. In 40 years of playing, I cannot tell you how many weaker players I have seen that carry around an arm full of opening books, but they never progress. You do not need to memorize openings when your opponent is not going to play them because he has not memorized them. You need to understand opening pawn structures. Chess Opening Essentials: The Complete Series (Volumes 1 - 4) is a very basic way to start this.

This is, of course, on a different level, but I recently read something about Kramnik training young prodigies, and one of them played an opening he saw a higher-rated GM play, and he imitated the play by rote, but soon went astray. He did not understand, at least on the much higher level he was at. Understanding is the key.

Beyond that, tactics are the most important thing you can do. Most chess games between weak players are decided by who drops the last piece, and these books would help you win more material, and thus, win more games.

I recommend starting with "1001 Brilliant Ways to Checkmate", and then "1001 Winning Chess Sacrifices and Combinations" (21st Century Editions) by Reinfeld.

I recommend doing 50 per day, spending no more than two minutes per move on each problem. In pencil, write down next to the problem how easy you found it on a scale of 1-5 because you will do these books multiple times. That immersion method has worked very well for me. I took a friend, who was about 38 at the time, from 1000 to 1850 on ICC in three months following this method.