column-rule

column-rule: dotted;
column-rule: solid 8px;
column-rule: solid blue;
column-rule: thick inset blue;

/* Global values */
column-rule: inherit;
column-rule: initial;
column-rule: revert;
column-rule: unset;

/* Same as "medium dotted currentcolor" */
p.foo { column-rule: dotted; }

/* Same as "medium solid blue" */
p.bar { column-rule: solid blue; }

/* Same as "8px solid currentcolor" */
p.baz { column-rule: solid 8px; }

p.abc { column-rule: thick inset blue; }

<p class="content-box">
  This is a bunch of text split into three columns.
  Take note of how the `column-rule` property is used
  to adjust the style, width, and color of the rule
  that appears between the columns.
</p>

.content-box {
  padding: 0.3em;
  background: #ff7;
  column-count: 3;
  column-rule: inset 2px #33f;
}