In terms of efficiency, "You fucking changed my CSS" says a lot in just a few words. Once that's out of the way, you can always get on to the finer points of why that's a bad thing.
I do agree that you should be able to say how you feel, but "You fucking changed my CSS" is an overreaction. I work on a project with 4 or 5 other people. Sometimes they change something and it breaks me, and sometimes I change things and it breaks them. When that happens I can choose to react in a few ways:
1) "You fucking broke my code."
2) "Why did you change my code?"
3) "Why you change THE code/Why did you make this change?"
4) Ignore it and fix it myself.
She could have stated, "Why did you change my copy? I carefully selected each word and I think the changes detract from the overall feel." Boom. Done. Dissatisfaction has been conveyed without sounding like a child.
Why choose words carefully when communicating to the public / investors, and not to team members / partners?
But trust me, do this for two years instead of just two weeks, and the team stops respecting or trusting each other. Each little verbal jab accumulates over time and deteriorates the working relationship. "Don't work with John or touch his stuff - he goes ape shit every time some little thing happens."
There's a good saying that goes "never attribute to malice what can be attributed to incompetence / stupidity / misunderstanding" (the last word depending on your context).
Basically, cursing someone out does not necessarily yield results over the long term when there is an underlying problem that can be solved in a more civil fashion.
I'm glad I haven't had to work with people who acted like this yet. As I'm pretty sure my response would be a very efficient "Fuck you, stop being a child." somewhere around the second time it happened.