Sadly, column boxes cannot be styled at present. The anonymous boxes that make up your columns can't be targeted in any way, meaning it isn't possible to change a box's background color or have one column larger than the others. Perhaps in future versions of the specification it might be. For now, however, we are able to change the spacing and add lines between columns.

The gap between columns is controlled using the column-gap property. This property was originally defined in the Multi-column Layout specification. However, it is now defined in Box Alignment in order to unify gaps between boxes in other specifications such as CSS Grid Layout.

The initial value of column-gap in multicol is 1em. This means your columns will not run into each other. In other layout methods, the initial value for column-gap is 0. If you use the keyword value "normal", the gap will be set to 1em.

You can change the gap by using any length unit as the value of column-gap. In the example below, the column-gap is set to 40px.

The allowed value for column-gap is a <length-percentage>, this means percentages are allowed. Percentage values for column-gap are calculated as a percentage of the width of the multicol container.

The specification defines column-rule-width, column-rule-style and column-rule-color, providing a shorthand column-rule. These properties work in exactly the same way as the border properties: any valid border-style can be used as a column-rule-style.

These properties are applied to the element which is the multicol container and therefore all columns will have the same rule. Rules are only drawn between columns and not on the outer edges. Rules should also only be drawn between columns which have content.

In this next example, a 5px-dotted rule with a color of rebeccapurple has been created using the longhand values.

Note that the rule itself does not take up any space: a wide rule will not push the columns apart to make space for the rule. Instead, the rule overlays the gap.

The example below uses a very wide rule of 40px and a 10px gap. The rule displays under the content of the columns. To make space on both sides of the rule, the gap would need to be increased to be larger than 40px.

This article details all the current ways in which column boxes can be styled. In the next guide, we will take a look at making elements inside a container span across all columns.