What you need to know
Custom properties centralize values that repeat across components. They can also change by theme or state without replacing every individual declaration.
How to apply it
Define tokens on :root for the base theme and override them on a theme scope. Name them by purpose, such as --color-surface or --space-section, and use fallbacks only where they add resilience.
Common mistake to avoid
Do not create a token for every one-off value. Keep the system small enough that a developer can understand which choices are intended to stay consistent.