Fix background color setting so it applies on the frontend

For original report, see:
https://github.com/cuny-academic-commons/cbox-theme/issues/88

When the dynamic stylesheet is compiled, switch 'background-color' rule to use the parent CSS 'background' rule instead.

This is so background color settings from the admin area are applied on the frontend instead of the 'background-image' rule taking precedence by default.

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Up to you guys if you want to apply this pull request as you might have other ideas on how to address this.