I hardly EVER eat in the theatre. Only if I'm SUPER SUPER hungry. And that RARELY happens. I'm the person who eats Sour Patch Kids, puts her feet up on the back of the seats, plays with the lights, and gossips in the movie theatre, so my information here isn't too valuable.
I know the last time I went to NYC, I saw Wicked and RENT. I wore jeans to both shows, but I only wore jeans to Wicked because I wore my Defy Gravity shirt and my green belt I bought JUST FOR that shirt. My shoes matched, too. (my special green and black Shox that are SO much more comfortable than the sherpa boots my mom made me bring...in OCTOBER)
For RENT, I wore a sweater with a hood, jeans, and the same shoes I wore to Wicked.
To see the Hairspray tour in January, I wore jeans and a nice sweater/sleeveless shirt combo with a scarf belt.
(Note: I add detail in the wrong places.)
Long story short, if I go to the theatre, I TRY to be comfortable, but also look nice. Even for the Regional/Non-Eq. place I work at AND attend regularly, I usually go for a dressy-casual vibe, because it's outdoors. (I wear skirts there all the time)
Every living soul has got a voice - you've got to give it room and let it sing.