Phase banner inverse option has a background set

[andysellick]

As part of this PR I've checked to see if any other components set a background colour for their inverse option - and the phase banner does.

Components with an inverse option should not set a background colour as the inverse option is designed to work against a dark background, and we can't always know exactly what colour that background will be (it's usually GOV.UK dark blue, but it might be a dark green or something else in the future).

To fix this we'll need to check and see if the phase banner is used anywhere with the inverse option, then test if its safe to remove the background colour from it (it's not a huge job but I don't have time to do it right now).