From this link: http://stackoverflow.com/questions/27411581/centering-no-captcha-recaptcha/29836372?sgp=2#29836372, I have learned how to use css to position No Captcha reCaptcha at horizonal center inside a page. Belows are sass-syntax code.

.your-recaptcha-class
  display: inline-block
  text-align: center

inline-block means to represent your element to be text-feature and block-feature. Its format wouldn’t generate a new line with other elements itself and it’s also to be configured with padding, width, heigth, background-image attributes, …, etc.

Extra reference about inline-block in Chinese: http://blog.xuite.net/vexed/tech/29221717-CSS+屬性+display+的值+inline+block+inline-block+none