Well, this is a first... Jon writing something I disagree with :-)
I think blue is a very nice color, and offsets the green of the woods behind the bike shed.
Seriously, the only possible complaint against Okular devs is what Jon said in a reply to a comment: make it easier for people to find that option.
Many programs have warnings which come up on first use or until you turn them off. Maybe something like that. And in kiosk mode they wouldn't come up, so a corporate deployment can happen.
I'm actually in the position of advocating a corporate deployment of Linux, so I certainly like the default, DRM-hater though I may be at home.