Moved background mixin to mixins directory.

[richardgaunt]

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• I have formatted the subject to include the issue number
  as [#123] Verb in past tense with a period at the end.
• I have provided information in the Changed section about WHY something was
  done if this was a bespoke implementation.
• I have performed a self-review of my code.
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Changed

1. Moved background mixin to mixins directory.

Screenshots