openings
Beyond opening principles? No, not really. Pick the opening you're most familiar with and every time you or your opponent deviates from what you know, learn from it. What would be the best response to your opponent's deviation?
This is something blitz games can be useful for as you are building the opening tree in your head rapidly by virtue of how many games you are playing. But beyond that, don't study openings too deeply. They have their place but they don't always decide a game, especially at the 400 elo level.
@1
"should i learn any theory on openings?" ++ No. Time and effort is better spent on endgames.
"what are some good openings" ++ Simplest and best for beginners and World Championship Candidates alike is to defend 1 e4 e5 and 1 d4 d5 as black and to open 1 e4 as white.
"to improve my games?"
++ To improve your games, analyse your lost games so as to learn from your mistakes.