definitely the attitude of the boss is extremely important. if the boss is an (beep) then no positive culture will ever be possible. sometime however part of the job to keep the culture and atmosphere positive is to take tough decisions.

I had cases when I had to part ways with high caliber professional staff only because there was serious risk of them becoming (or some managed to become) rotten apples that totally disturbed the workings of the office. one person like that can destroy years of building what you call here positive culture. sometime have to let that kind of people go even though from professional point view their departure is a loss to the company