message: style(Field): adjust placeholder color to gray-6
diff: @field-input-text-color: @text-color;
  @field-input-error-text-color: @red;
  @field-input-disabled-text-color: @gray-5;
  -@field-placeholder-text-color: @gray-5;
  +@field-placeholder-text-color: @gray-6;
  @field-icon-size: 16px;
  @field-clear-icon-size: 16px;
  @field-clear-icon-color: @gray-5;
  