Spacing
Sandbox adds additional spacing variables to existing Bootstrap spacing ranging between values 0 and 25
Font Size
Quickly change the font-size of text using .fs-* classes ranging between 1 and 200, while sm, and lg also apply.
Line Height
Change the line height with .lh-* utilities.
Letter Spacing
Change the letter spacing with .ls-* utilities.
Gutters
Spacing variables mentioned above can also be used for column gutters. More information regarding Bootstrap gutters can be found on Bootstrap docs
Retina Images
In order to display a high resolution image on devices with retina display, you need two versions of the image. One with regular size and another with twice its size. The high resolution one should have the exact same name with @2x at the end.