Use variables with fallbacks for utility classes

[adamwathan]

This PR makes utility classes use variables with fallbacks for their underlying values, rather than just using the bare value:

/* Before */
.pt-4 {
  padding-top: 1rem;
}

/* After */
.pt-4 {
  padding-top: var(--spacing-4, 1rem);
}

Not totally sold on it yet but opening as a draft as a reminder to discuss with the team next week.