code: public CssStyle setProperty(final String key, final String value) {
      if (this.properties == null) {
        this.properties = new HashMap<String, String>();
      }
      this.properties.put(sanitizeCssPropertyName(key), value);
      return this;
    }
docs: Sets a CSS property.
  
  @param key
  the name of the property. Note that hyphen notation ("-") is
  automatically converted into camel case notation since the
  property name is used as key in a JSON object. Highcharts will
  evaluate it as if it were hyphen notation.
  @param value
  the value of the CSS property.
  @return this for chaining.