code: @Nonnull
    public ExcelStyle setFont (@Nonnull final Font aFont)
    {
      ValueEnforcer.notNull (aFont, "Font");
      return setFontIndex (aFont.getIndexAsInt ());
    }
docs: Set the index of the font to use. The font must have been previously
  created via Workbook.createFont()!
  
  @param aFont
  The font to use. May not be <code>null</code>.
  @return this