Not sure what to do?

"All that he said was he doesn't want to do anything for his birthday"

And... you failed to listen to that, ignored his request, and invaded his personal space to boot.

Is he overreacting? Possibly. Does he have something to react to? Hell yeah.

You have no idea whatsoever what people are dealing with on a birthday.

None.

He could have lost a beloved family member or close friend on his birthday, or when they were the age he has now become. Or maybe he doesn't want to be reminded of the aging process. Or maybe his faith (Jehovah's Witnesses, for example) doesn't permit such celebrations. Or he thinks it's ostentatious. Or he thinks it's inappropriate at work (which it is; you're not kids in first grade; you're alleged professionals in a business).

His reasons for not wanting to celebrate do not matter. They are not your concern, nor are they your business. If he tells you his reasons, it's not your call whether those reasons are good enough, exaggerated, or anything else.

He said no. You went ahead anyway.

I'd be pissed at you, too.

As for what you should do, apologize like the grownup I hope you are. No other conversation; you're saying you're sorry and nothing else. No explaining, no waffling, no pleading. Your speech is exactly two words long: I'm sorry.

Chalk it up to differences in customs and think twice before ignoring people's express instructions again.

you need to take responsibility for your inappropriate action
you should apologize

you need to learn to listen to what people are saying and respect their wishes

Screw it!! You did something nice for him..decorating his cubie for his birthday. He needs to get off his high horse..you owe him no apologies whatsoever!!