code: public void setTheme(final JavaScriptResourceReference theme) {
          if (this.theme != null || this.themeUrl != null) {
              throw new IllegalStateException(
                      "A theme can only be defined once. Calling different setTheme methods is not allowed!");
          }
          this.themeReference = theme;
      }
docs: Sets the theme for this chart by specifying a reference to a javascript
  file containing the theme. The javascript file must contain the following
  code: <pre><code>
  Highcharts.setOptions(myOptions);
  </code></pre> where <code>myOptions</code> is a JSON representation of the
  theme options.
  
  A theme can only be set via one setTheme method. An
  {@link IllegalStateException} will be thrown if you call two setTheme
  methods.
  
  @param theme reference to a javascript file containing a theme.
  @see <a
  href="http://www.highcharts.com/demo">http://www.highcharts.com/demo</a>