Is it possible to assign custom Keyboard Shortcuts to Styles in Pages 5.0?
I'm afraid there is no way to do it. Apple apparently removed the feature and it is also not possible to do it via the OS X shortcuts because there is no Menu entry for choosing styles in Pages.

Here's a discussion in the Apple Support Communities where people are pretty upset about this truly annoying feature regression: https://discussions.apple.com/message/23628178#23628178