They need to update the iWork suite as a whole. All the iCloud bits have the new UI (Mail, Calendar, Contacts, Reminders, Find my Phone) but the iWork bits all have old UI.
Maybe it's going to be an overhaul of the iWork UI across all their systems?
It's strange they created it using the old look when it was presumably in development around the same time as iOS 7. I imagine the iWork team saw iOS 7 announced at the keynote and though "shit, why did nobody tell us?".
Judging by class names in the div soup of iWork webapp it supports theming, so presumably they'll flip the switch "when it's ready".
If they chose any time to update the iWork suite, then it should be now. iWork for Mac hasn't seen an update since 2009, and both iOS and OS X are very different landscapes since then.
In the keynote, they mentioned redoing the entire iWork suite later in the Fall.
Did they? I don't remember that. Did they include the OSX suite in that, or just iOS?
Been a while since I saw it, but I believe they only mentioned something like that there would be "a new release of iWork this year". No specifics about how major it'll be (still "iWork '09"?) or whether that was OS X, iOS, or both.