Blundering is the biggest issue sub candidate master, or so I have heard. Even principles can not stop blunders because they can be less than obvious at times, other times very obvious, but still the biggest reason for a loss at any level.
I switched my opening repertoire to opening principles

Yeah, opening prep genuinely doesn’t matter. I create an entirely new repertoire to play at least twice a month and I still hover 1700-1850 consistently. All I have yet to play full-time are the Catalan, English, English Defense, Tarrasch, QGA, Benoni, Blumenfeld, Dutch, Petroff, Philidor, and Polish and I’ll have played every single reputable opening I’m aware of.

Prep doesn't matter. But as you improve your spidy senses tingle and you get better in other aspects of your game which you previously didn't. Obviously this doesn't come automatically you have to play, analyse and learn a lot.

Blundering is the biggest issue sub candidate master, or so I have heard. Even principles can not stop blunders because they can be less than obvious at times, other times very obvious, but still the biggest reason for a loss at any level.
Sure we can all stand to get better board vision and calculation skills, but tactics flow from good positions, and some opening theory can help with getting good positions or at least avoiding bad ones. Love it or hate it (I mostly hate it), theory is part of the game. It should neither be overtrained nor completely ignored.
@5
If you depend on theory to avoid bad positions in the opening,
then how will you avoid bad positions in the middle game?

@5
If you depend on theory to avoid bad positions in the opening,
then how will you avoid bad positions in the middle game?
You probably think this is a clever quip, but it misses a crucial difference. In the opening I am playing against my opponent's opening theory. In the middlegame I am playing against my opponent's ability.
No one plays better than theory. Not me, not you, not Magnus Carlsen. Not even pocket stockfish. Why? Because theory is a combination of hundreds of years of human experience and the most powerful analysis that the best chess players can perform with the strongest engines. If you're on your own playing against theory you will be outplayed. It's that simple.
Now, don't get my little rant misconstrued. No one should try to memorize their way to success. That way leads to failure. A much worse player can out-memorize me get a good position and then throw it away once we're both playing our own moves. The point though, is at a certain point you need to know a bit of theory to avoid issues in the opening. And that point is certainly well below the CM level.
So to answer, your question? If I rely on theory to avoid bad positions in the opening, how will I avoid bad positions in the middle game?
By playing human players who make mistakes just like me, instead of bulletproof prep.
@7
"If you're on your own playing against theory you will be outplayed."
++ No. There are many counterexamples. Here is one:
https://www.chessgames.com/perl/chessgame?gid=1139797
On move 19 Honfi had used 5 minutes and hence was clearly following theory / analysis.
Tal had used 90 minutes and hence was clearly on his own.
The stronger player Tal outplayed the better theoretician Honfi.

@5
If you depend on theory to avoid bad positions in the opening,
then how will you avoid bad positions in the middle game?
You probably think this is a clever quip, but it misses a crucial difference. In the opening I am playing against my opponent's opening theory. In the middlegame I am playing against my opponent's ability.
No one plays better than theory. Not me, not you, not Magnus Carlsen. Not even pocket stockfish. Why? Because theory is a combination of hundreds of years of human experience and the most powerful analysis that the best chess players can perform with the strongest engines. If you're on your own playing against theory you will be outplayed. It's that simple.
Now, don't get my little rant misconstrued. No one should try to memorize their way to success. That way leads to failure. A much worse player can out-memorize me get a good position and then throw it away once we're both playing our own moves. The point though, is at a certain point you need to know a bit of theory to avoid issues in the opening. And that point is certainly well below the CM level.
So to answer, your question? If I rely on theory to avoid bad positions in the opening, how will I avoid bad positions in the middle game?
By playing human players who make mistakes just like me, instead of bulletproof prep.
Ehhh… I remain unconvinced. My first example would be unorthodox first opening moves, like who preps against 1. a3, 1. c3, 1. d3, 1. e3, 1. Nc3, or 1. h3? And yet, there’s no advantage to be had for Black.
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I got that free subscription for chessmood and decided to stop playing my opening prep until the subscription is over. I expected to drop to 1300, then climb back to 1500 using the knowledge of the website and, after returning to my opening prep, get another 100-200 elo on top. You can imagine my surprise after breaking the 1500 wall the very next session. (now my rating is 1545 but it is that big only because one guy spammed rematch 8 times). I noticed that people just blunder or develop incorrectly in e4 e5 positions very often. Yes, I got KILLED, DEMOLISHED and SLAUGHTERED by that Ng5 scotch game trap, but that was only one game. It's just crazy for me that no opening prep worked better than my semi perfect french defence repertoire. Here are some examples of the games where my opponents played the position not well