Hi my advice is that if you don't know the theory to play, just follow the basic principles of occupy the centre, develop your pieces and castle your king
Strategy when opponent does something unexpected in opening

Check for basic tactics that arise from the position. There might be a little trick you haven't seen before to win a piece or a pawn. If the position looks neutral then just develop your pieces, control the centre and get your king safe.
@1
"Strategy when opponent does something unexpected in opening" ++ Think!
"what’s the plan when you don’t recognise an opening?" ++ Think!
"Do you just play toward general principles?" ++ Yes, but with concrete calculation.
"So as soon as they move from the well known lines I can probably just expect that they don’t know the theory and I’m at a slight advantage already?" ++ Not necessarily. The unexpected move can be equal, worse, or even better that what you know. You have to think. What are the advantages and the disadvantages of his move? Can you exploit it in some way?
Two examples:
1...a6 was unexpected
https://www.chessgames.com/perl/chessgame?gid=1068157
8...d5 was unexpected
https://www.chessgames.com/perl/chessgame?gid=1095025
Now, I’m very much a beginner, so there will be a lot of very valid chess openings I won’t have seen before/be prepared for, but what’s the plan when you don’t recognise an opening?
Do you just play toward general principles?
This obviously within the context that I’m playing players of similar ability, 800-1200 currently, and they’ll be as unlikely as me to be playing some obscure opening.
So as soon as they move from the well known lines I can probably just expect that they don’t know the theory and I’m at a slight advantage already?