Themes for stylized node-vega and node-vega-lite visualizations

 A Vega *theme* is a configuration object with default settings for a variety
 of visual properties such as colors, typefaces, line widths and spacing. This
 module exports a set of named themes, which can be passed as input to the
 node-vega or node-vega-lite with node-vega-embed or directly as a
 configuration object to the Vega parser.