The Paris opera may be a special occasion for you and you will want to look nice, but my best advice is to be careful of your attire but don’t overdress unless it is the opening night of a grand gala and there will be movie stars and politicians and the like attending. Even then the French tend to be conservative, so no supper low cut or over the top outfits. Men would wear a tux on such a rare occasion.
The dress for a regular night would be more casual. Men can wear a jacket but most do not wear a tie. You will see both men and women in business dress. For a matinee they are very often coming straight from work. Quite often men wear a dress shirt, but some wear a polo shirt or even a sweater and nice pants and shoes. You will even see nice black jeans on some Parisians but especially on tourists.
In a summer evening women would look great in a simple street legnth dress with a light jacket or shawl over it. Even a casual midi dress would be fine. For footwear a fancy sandle or ballet flat would work.
Take a nice jacket or coat with you, and check it or keep it with you. If you have packed a trench coat that would be suitable. What about one of those new casual capes…of course you might not have room for something like that if you packed light.
The covered perfectly black dress is easy to pack and would look lovely.
You probably would be fine if you dressed like you would for the opera in your home town unless you live somewhere that requires men to always wear a suit and tie and ladies to wear gowns. This is not the norm at the Paris opera.
Don't fret too much as long as your well groomed and dress conservative you will fit in. Relax and enjoy the evening.
I have found some nice ensemble ideas to help you look wonderful and appropriate, and we will look forward to seeing your pictures when you get back.