Fix global styles for unbranded layout

[lfdebrux]

I noticed that one of the text lines wasn't formatted correctly on the password page, because it was missing the govuk-body class. Digging deeper, it seems that we used to expect that the GOV.UK frontend global styles would work for pages using the 'unbranded' stylesheet, but it stopped in the rework for v13.

The issue is that we import the govuk-frontend settings in the unbranded template, before we set the govuk-global-styles variable to true. Previously this was fine because we were setting that variable strongly, however in v13 we added the !default thingy [1], because we want users to be able to override the value in settings.scss. I think this behaviour is unavoidable, but luckily there is a workaround; we were only importing the settings to get govuk-body-background-colour, but the value of that is just #fffff (white). We can set the value to white without needing to import any settings. If the GOV.UK Design System decides to go with an off-white body background colour in the future, that's also fine, the point of the unbranded template is not to look like GOV.UK, so we're not losing anything by hardcoding the value.

[lfdebrux]

Another option that feels less hacky is to copy what the GOV.UK Frontend review app does, and have a CSS class to remove the footer:

# unbranded.html

...

{% set htmlClasses = "app-no-canvas-background" %}

...

# unbranded.sass

@import "prototype.scss"

.app-no-canvas-background {
  background-color: $govuk-body-background-colour;
}

https://github.com/alphagov/govuk-frontend/search?q=app-no-canvas-background

The advantage of this is that it can go after the import of GOV.UK Frontend Sass, so the constant $govuk-body-background-colour will get imported in the way it is supposed to rather than us having to import by hand.

The fact that this wasn't done originally though suggests that maybe this isn't a complete solution? Not sure 🤔

[lfdebrux]

Discussed with @joelanman , we agreed that we want the background to be white and don't care about what the value of $govuk-body-background-colour is, so we can sidestep this issue. Will update the PR now.

[joelanman]

thanks!