CSS branding not applied to sign in page (Help Center)

We brand our Help Center with CSS. This works fine for the home page and the "submit a request" page.

The branding isn't applied to the "Sign in" page:
http://XXX.zendesk.com/access/unauthenticated?return_to=%2Fhc%2Fen-us&theme=hc

Zendesk Support told us (ticket #498068) that this feature is currently missing. I do believe it's very important for a consistent, professional experience towards our customers.