E4 4 me
D4 or E4?
Don't bother with opening repertoires. Learn opening principles and just a few first moves of openings you play. Concentrate on tactics instead.
Here:

You want some positional openings that improves your position?d4 saves the day!
#1
"I don't know if this would be a good choice though, and if I should instead only pick one."
++ Pick one and stick to it so as to accumulate experience with it.
"if you also have any advice for an opening repertoire for both white and black"
++ As black defend 1 e4 e5 and 1 d4 d5 and as white open 1 e4
"I have been trying to learn the Danish gambit for white"
++ Objectively Danish Gambit is bad, but you can play it at your level.
"But I haven't learned anything for black yet"
++ You do not need to learn, just practice and learn as you play.
Black is more important than white. As black you have less room for error than as white.


Start with e4. I think its the more principled of the two as it helps you focus on quick development and castling. And while I agree sticking with one sort of opening, I think there is merit in playing all sorts of structures which arise from different openings.
Also, if you're going to pick openings, pls don't pick the rubbish gambit openings, stick to the classics. Ruy Lopez, Italian, Vienna etc. You wont win rating points as quickly as if you played the tricky openings like the Danish, but it'll help you improve in the long run.
I am a sort of new to chess, and am trying to get better at my openings. I preferably really like playing the e4 square for white, but haven't looked much at black yet. I have heard that if you really want to get better at chess, you have to pick one square and master it. But at the same time, I feel like I should learn both e4 and d4 squares so I can get used to both, just in case I need the other. I don't know if this would be a good choice though, and if I should instead only pick one. This could all effect my repertoire depending what I should do, so if you also have any advice for an opening repertoire for both white and black, that would be great! (Ps. I haven't learned any openings thoroughly, but I have been trying to learn the Danish gambit for white, and really like it. But I haven't learned anything for black yet). Sorry if this was very confusing, but if you could help me at all, that would be great! Thank you for your time, and have a great rest of your day!