Can you bring a backpack in the theater?

I took a large bag, and that was fine. They will look into it, and might ask you to go through it, so a smaller backpack would probably be easier. (That being said, calling and asking them directly would be the best choice if no one else knows for sure backpacks are ok.)

Yes they just check it when u enter

i brought in my 30L backpack. they will check and thats all.

Yes.