My sources of opening theory are mostly focuses on things to do in it, and some ideas about why.
What are good sources about things to avoid, and ideas that can't be developed in an opening?
This site has some material. Things like don't block your center pawns with your pieces, and don't play into fool's mates, etc.
Then there is a series of beginner-ish books from Pandolfini: Chess Openings: Traps And Zaps
also, Win in the Opening: Opening Mistakes and How to Punish Them by Yakov Neishtadt
Perhaps Taming Wild Chess Openings, written by IM Watson and FM Schiller, is interesting for you. The book contains a collection of bad, dubious and tricky openings. The authors show how to react and, if possible, refute them.
Just look at a bunch of master games (300 or so; maybe 100 per day) and you'll recognize the patterns.
I'd say just look at the first 15 moves and no more. Take no longer than five minutes a game.
When you're really good, you'll be able to take one minute per game. Remember, you're only looking at the first fifteen moves. The patterns are always the same.