I have this problem on 2 blogs. I've upgraded one & the other is a new install.
Basically, the header doesn't show. The blog title text is there, but is white, on a white background. I believe, in Kubrick, that bit of background should be blue?
I've played around with it, and I can change the header text colour fine. But I can't do anything to the header background. I even tried uploading an image header (and changed it from being commented out - yup I spotted that, at least!) but it wouldn't even display that.
BTW, see leroc.net/blog to see what I'm on about.