That sort of clause is pretty much universally challenged by anyone with a bit of experience and credibility, too, at least here in the UK. And so it should be. That goes whether we're talking about inventions (which normally means patents) or other intellectual property such as copyright (which appears to be what we're talking about in OP's situation).
"In the course of your employment" or similar words used to be a more reasonable standard that was widely considered fair to both sides for salaried employees. In other words, things you do on company time, using company resources and/or in the company's line of business go to the company, but they have no claim beyond that. I haven't spoken to a lawyer about this recently, so if the issue might affect you, please check whether this is still correct and don't just rely on my comment here.