CSS custom properties (variables) can pierce the Shadow DOM boundary of web components, allowing external styling without modifying the component internals. By defining custom properties with fallback values inside a web component's shadow styles (e.g., `var(--alert-bg, rgb(136 177 255 / 0.5))`), consumers can override visual appearance by setting those properties on the host element from outside — enabling theming variants like error and success states without touching the component's code.
5 Impressions