create-theme.js

var themeObject = {
  name: 'custom',
  theme: {
    base: {
      chart: {
        showlegend: '1' 
      },
      dataset: [{
          data: [{
            color: '#ff0000'
          }]
      }]
    }
  }
};

// Now we create a theme extension with the above theme object
var theme = {
  extension: themeObject,
  name: 'customTheme',
  type: 'theme'
};

// We are now required to add the theme extension as a dependency to fusioncharts
FusionCharts.addDep(theme);