[Bootstrap 3] 格狀系統斷點 Break point
更新日期:參考：https://getbootstrap.com/css/#grid
/* Extra small devices (phones, less than 768px) */
/* No media query since this is the default in Bootstrap */
/* Small devices (tablets, 768px and up) */
@media (min-width: @screen-sm-min) { ... }
/* Medium devices (desktops, 992px and up) */
@media (min-width: @screen-md-min) { ... }
/* Large devices (large desktops, 1200px and up) */
@media (min-width: @screen-lg-min) { ... }
其中
@screen-sm-min 為768px
@screen-md-min 為992px
@screen-lg-min 為1200px