the top should have a button to hook the hood of the gown on to. Is that really necessary?
Yes, yes, yes.
Academic gowns are designed for men. The hood is made with a loop which loops around a shirt button under the tie. So wear a suit or a dress with buttons down the front. Or make sure you have a VERY strong safety pin.
Tie your hair back so it's not all over the place under your mortar board. Wear your mortar board flat not at the back of your head.
Don't wear shoes with too high a heel, as you will invariably need to climb stairs and walk across a podium in front of hundreds of people. Wear a dress with an elegant skirt length, not a mini skirt.
Black sounds OK as your gown is not black, but I'd wear a jacket, so you have long sleeves coming through the gown. Honestly, you're likely to feel underdressed in bare-ish arms.
I have to sit through two graduations a year & I'm sick of seeing sloppy trousers & untucked shirts on the men, and night club style attire on the women. It's a serious wonderful occasion - treat it with respect. It's a great excuse for looking for a really elegant smart dress or suit. All the guests there will be dressed formally, as will the academic staff on the podium - my male colleagues are usually in sub fusc, but I get away with a skirt suit.
In my day we had to wear white dress & white gloves and curtsey to the Chancellor!