Adjust theming acceptance tests to new header colour transition

Before, the acceptance tests checked the header colour just once, as the
header colour was immediately changed once the new theming colour was
saved. This is no longer the case, as currently a transition is used to
change between the original colour and the new one, so now the
acceptance tests check repeteadly for the expected header colour until
it matches or the timeout expires.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>