Ensure sign-in page background is uniform throughout the page (#1988)

* Ensure sign-in page background is uniform throughout the page

Configured banners that take up large amounts of space leave a gap of blank
background between where the body ends and the footer starts.  Fix this by
setting the style for the section containing the banner to match the body and
footer

* Add changelog entry

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Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>