The culture at my workplace has been very negative for a long time but it's starting to turn around. The CEO finally realised how much money the business lost annually because of high employee turnover and the consequent loss of knowledge and momentum.
We've now hired some business coaches to help us instill a new culture and practice. It's still early in the process but it's already helping. One of the key things they've asked us to do is avoid "playing below the line", i.e. negative talk, blame, gossip, all the stuff that doesn't bring a more productive workplace along with permission for everyone to be able to challenge whenever they think someone is playing below the line. Another is "to always assume positive intent", i.e. don't immediately view anything that's said or done as negative.
There's lots more to it and it's a complex web of initiatives, and it's a slow process to turn around ingrained culture. It would not be easy to do any of this alone so I'd suggest you find some like-minded individuals who want to work in a better environment so you can collectively start to make it happen.