Common bar chart style overrides.
Override default x-label title placement (i.e. to account for long labels). Default 40px.
  chart.override = {
    "axes": [
      {
        "titleOffset": 200
      }
    ]
  };

Override default default chart color (#256e9d).
The following sets the main color (update) to light green, and the hover style to dark green.
  chart.override = {
    "marks": [
      {"properties": 
        {
          "hover": {"fill": {"value": "#17a086"}},
          "update": {"fill": {"value": "#7fcdbb"}}
        }
      }
    ]
  };

For for further chart defaults, inspect the template. Modify these defaults in chart.override.
  var tmpl = chart.specification.template;