Reusable classes for grid-column or grid-row span

Hey everyone!

(This isn’t specific to a site, so no readable link to share.)

I’m trying to improve my css reusability (word‽) and am having trouble.

Normally, I can write:

.span-12 {
    grid-column: span 12;
}

Thus creating reusable classes for grid items. However, I’m having difficulty replicating that line in Webflow. Is this a situation for a custom code embed, or am I too thick and overlooked something?

Thanks in advance!
Colin