code: public static void registerVariant(String name, Style style, String variantName) {
      Map<String,String> styleVariants = _variants.get(style);
      if (styleVariants == null) {
        _variants.put(style, styleVariants = new HashMap<String,String>());
      }
      styleVariants.put(name, variantName);
    }
docs: Registers a font for use when a bold, italic or bold italic variant is requested. iOS does not
  programmatically generate bold, italic and bold italic variants of fonts. Instead it uses the
  actual bold, italic or bold italic variant of the font provided by the original designer.
  
  <p> The built-in iOS fonts (Helvetica, Courier) have already had their variants mapped, but if
  you add custom fonts to your game, you will need to register variants for the bold, italic or
  bold italic versions if you intend to make use of them. </p>
  
  <p> Alternatively, you can simply request a font variant by name (e.g. {@code
  graphics().createFont("Arial Bold Italic", Font.Style.PLAIN, 16)}) to use a specific font
  variant directly. This variant mapping process exists only to simplify cross-platform
  development. </p>