Update 3 files

Daniel Cothranrequested to merge
issue/charts-3609779:3609779-the-background-color into 5.2.x
  • /modules/charts_chartjs/src/Plugin/chart/Library/Chartjs.php
  • /modules/charts_billboard/src/Plugin/chart/Library/Billboard.php
  • /modules/charts_c3/src/Plugin/chart/Library/C3.php

Closes #3609779

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