Replace 'unset' property in printed background colours

[matthillco]

What

In improving print styles in gem components (#4073), we used background-color: unset to revert backgrounds to transparent for print. This would ensure that inverse components would always print non-inverted, saving ink.

After discussion we decided to change this to use background: none instead, because:

• the use of unset is uncommon and not well understood, and its behaviour can be unpredictable
• background: none is a more common accepted approach and easier to understand. We've taken into account that resetting all background properties can be considered a CSS anti-pattern, however we're confident in this case that it won't have any adverse effects because we're not using any other background properties in the affected components.

This PR also makes consistent other treatments of background, such as background-color: transparent or background: initial.

Trello

[AshGDS]

LGTM 👍