Twenty Fifteen: Don't save unfiltered CSS in a setting.

Introduce `twentyfifteen_get_color_scheme_css( $colors )` which will be used for Underscore/Backbone and the PHP side. Let `twentyfifteen_color_scheme_css()` handle the color calculation on PHP side.

Merges [30893] to the 4.1 branch.

Fixes #30409.
Props ocean90

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