Chess Position Trainer. The free version of CPT5 is full for a month and then is a trial version there after, but the trial version is plenty useful for what you want to do.
Or you could get the old 100% free CPT3 from my website http://gorgonian.weebly.com/pgn.html
(since it is hard to find nowadays)
As a beginner I am trying to master standard openings but playing online or with a PC usually involves the opponent making non-standard moves at times.
Welcome to practical chess. Only Experts and Masters will gladly skip along holding your hand down a trail of book lines 15-20 moves deep. Everyone else will deviate ... some quite early.
Your own experience is already strongly hinting (or perhaps even yelling!) that this is a far less efficient way to study openings with very little return on investment, especially if you are a beginner.
Focus on opening systems or better yet, grab a beginner-facing instructional + annotated classic game collection (Logical Chess by Chernev, Chess Master vs. Amateur by Euwe are excellent examples.) and you'll be able to survive the opening at your current level without bothering with one ounce of book opening knowledge.
The following clickable website will take you to a software program system known as Chess Openings Wizard which used to be called Bookup:
https://www.facebook.com/ChessOpeningsWizard
The ChessBase organization also has something similar.
As a beginner I am trying to master standard openings but playing online or with a PC usually involves the opponent making non-standard moves at times.
Is there a program out there where I could pick the opponent's opening play?