I am a 1400-rated player currently working on my theory and understanding of openings.
I am good at learning lines, much more so than others at my rating. But I'm less sure on where/how to learn the theory behind my preferred openings (I prefer the London System as White, and the Taimanov Sicilian as Black). Therefore, I don't know how to capitalize when my opponent doesn't know the lines themselves.
For example, I know the main line of the Taimanov Sicilian to the 18th move, as well as how to transpose into several variations along the way depending on White's move. However, what I struggle with is knowing what to do when my opponent plays a move that is not part of any studied line.
To display this, in the Taimanov Sicilian, the standard line is 14. Ne4 O-O 15. Bc5 Bb7; if white played a move other than 15. Bc5 (that wasn't an obvious blunder or mistake), I would struggle to know how to continue.
I guess what I'm essentially asking is how and/or where I can learn the theory behind the openings. Every video I've found simply walks through the main lines and standard variations, and Wikipedia articles generally only give one-sentence summaries of the theory behind certain openings (for example, their section on the Taimanov Sicilian reads simply "Black develops the knight to a natural square and keeps his options open regarding the placement of his other pieces. One of the ideas of this system is to develop the king's bishop to b4 or c5."