chart backgound and text colors

@tomwayson Starting this conversation up because we need to make decisions on this. This will be covering text, background, and chart colors. Chart colors will need to be thought of in the context of multiseries as well.

Previously I'd come up with something like: `styles: { background: { color: '#xyz', alpha: 0.3 } }`
`alpha` is needed for background color because it defaults to 0, completely transparent.

That being said AmCharts docs say: `Background color. You should set backgroundAlpha to >0 value in order background to be visible. We recommend setting background color directly on a chart's DIV instead of using this property.` <--- makes me think we should re-consider how we handle this.

General thoughts @tomwayson ? (previous discussion is located at: https://github.com/Esri/cedar/pull/382