The size of the shadow. If X and Y offsets are set to 0, setting a positive spread will result in an even shadow on all sides. Negative values are also permitted.
Value
Description
(numeric)
e.g. '20' would create a 20 pixel shadow surrounding the element (Microthemer automatically adds the 'px' unit if a unit isn't specified).
Box Shadow Spread
The size of the shadow. If X and Y offsets are set to 0, setting a positive spread will result in an even shadow on all sides. Negative values are also permitted.
Themeover CSS Reference
Online CSS References: Box Shadow Spread
Online CSS Tutorials: Box Shadow