The solution is in the question itself. Take the css out of the content. There are hundreds of good reasons to avoid putting css into a node and you’ve just found a new one. The real question is what are you trying to achieve with the css being located in the node? One work around if you really need per node theming might be to use the css injector module. I’ve never used that module, but it’s description makes it seem like it could be a fit.