Someone in your organization needs to be in charge of frontend fidelity. I don't mean an official job title, I just mean someone who has taken it upon themselves to have a "the buck stops here" mentality—better yet someone who is recognized by the rest of the team to have that priority.
If nobody else fits the bill, then that person is you. And by all means, make all the stink you want about these issues. Nobody should ever be touching global stylesheets that affect multiple components or screens throughout the system without there being subsequent review or issues filed in a very visible way. Ideally, those sorts of breaking changes would never make it through code review in the first place.