Support custom color name in the config

We should be able to define:

config: {
  color: {
    "uw-purple": "#XXXXXX"
  }
}

and then use "fill": {"value": "uw-purple"} in the code.

This will be especially useful to create a visualization that works across themes. For example, one can name a color by role (such as “highlight”) and then set the “highlight” color to be different color for different themes.

cc: @Saba9

[kanitw]

@jheer Yep, this seems like the right direction for Vega.

[jheer]

Thanks @kanitw. I’m interpreting your comments above to state that you support this direction for Vega, but more work is needed to determine how to surface it in Vega-Lite. Is that correct, or should I wait on this issue for a while to see if there are other concerns that Vega might address? (If we want to keep config files interchangeable between Vega and VL, that means signal definitions might be present in VL config files… of course, I think this is already true in theory given Vega’s existing support for signal-valued config entries.)