Property Reference - CSS Reference - page 177

Name
column-rule-width
Values:
thin | medium | thick | <length>
Initial value:
medium
Applies to:
Elements laid out using multiple columns
Inherited:
No
Computed value:
Absolute length; 0 if the style of the border is none or hidden
Description:
Defines the width of the “rules” (vertical lines) drawn between columns in an element laid out
with multiple columns.
Examples:
#d01 {column-rule-width: 5px;}
#d02 {column-rule-width: 2em;}