Novice - Capitalizing on players not knowing their openings? 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."
I think your problem comes from the difference between memorizing the opening theory, and understanding it. I suggest you focus more on understanding, why this move is played (both yours and theirs). This way, you can think during the game, why this move could be not optimal and play accordingly. Even if you do not know, how to take advantage of it, you can it least follow the theme of your opening.

To understand the theme of an opening, you can (most of the times) read books about it. The introduction will contain the goals of the opening most of the times. You can also just take a chess board and look for yourself, what the lines of the openings are about.

To understand the reasons behind the moves, just play out different moves, which look natural to you. I recommand not to use an engine at first, and only check afterwards, if you missed something. Do not memorize every line, but rather look for common motives, which appear, e.g. they do not defend a field, so now you can put your knight there which puts pressure on a crucial field. Or they trade DSB, so now they have a weakness on dark squares (if pawns are on light squares), so you can try to capitalize on that. 

My personal experience however is, that at your level (and some levels above) gaining minimal advantages through the opening is not so important. A lot of games will be decided by tactics, so remember to constantly look out for them!