I am not as high rated as you but have found "Winning in the Opening" by John Walker a pretty decent book. If you can solve all the puzzles and catch all the insights in the games, kudos to you my friend.
studying opening theory

To reference Andrew Soltis's book 'The wisest things ever said about chess' - #243 - Until you are 1800, your first name is tactics, your middle name is tactics and your last name is tactics.
In reality you can do whatever you want and the effectiveness of studying theory vs practising tactics is really subjective. Some find one better than the other. In the end you learn more chess! :)
From my personal experience, I started studying theory at around your level. I can get a positional play but not much tactical play. Which I'm trying to change. :b
The book I mentioned has the following: (it sort of is geared towards beginners I guess)
64 puzzles
10 basic principles
~10 tactical themes and ~10-15 games showing them
analysis of 34 other games
---
All in all, 126 pages
I'll post a puzzle from the 'easier section'

As long your opening study involves looking at alot of themes (tactical and positional themes, like Nc6-b8-d7 in the ruy lopez or ...c4 in the modern benoni, I see no problems studying openings.

When you fancy it. It can be useful at any stage, it's all about balance.

When is a good time to study opening theory? At what level is the right time? I am familiar with basic opening principles. I usually spend my time studying strategy, endgame, and tactics and consider myself familiar with those phases of the game. In real life (reality) I would be rated around 1400-1500, should I start studying opening theory now?
Maybe you have to know some of the ideas for the most popular openings, and some lines about the openings you have in your repertoire ! But I don't think you have to memorize very long lines of 20-25 moves at your level!

To reference Andrew Soltis's book 'The wisest things ever said about chess' - #243 - Until you are 1800, your first name is tactics, your middle name is tactics and your last name is tactics.
In reality you can do whatever you want and the effectiveness of studying theory vs practising tactics is really subjective. Some find one better than the other. In the end you learn more chess! :)
From my personal experience, I started studying theory at around your level. I can get a positional play but not much tactical play. Which I'm trying to change. :b
But what good are tactics if you don't know what to look for? You need posional understaning as well to properly evaluate positions when calculaang tactics. If a tactic leads to a long term advantage where you can exploit a weak color complex or establish an eternal knight on the fifth or sixth the you'll consider such tactics that do so.

Whenever you want, chess is a game.
Chess is more than just game, chess is life and I don't want to screw it up.

First half of that statement had me intrigued, second half makes me wonder.
How come?
Is there a difference between studying openings and studying opening theory?
Yes! Opening theory is about the search for a white advantage. Studying openings is learning how to play the opening.
For instance, opening theory will tell you that the position after 1.d4 d5 2.c4 e6 3.Nf3 Nf6 4.Nc3 c6 5.cxd5 exd5 is equal, and probably give a line or two as well. And then the book stops, because it's not interesting as far as opening theory is concerned.
But you better know how to play such positions! I think Sadler in his QGD book says that he kept losing such positions as black "as a young IM".
You study classical games and pawn structures to learn to play that opening well, besides knowing the opening theory.
When is a good time to study opening theory? At what level is the right time? I am familiar with basic opening principles. I usually spend my time studying strategy, endgame, and tactics and consider myself familiar with those phases of the game. In real life (reality) I would be rated around 1400-1500, should I start studying opening theory now?