support custom property fallback values

[leeoniya]

https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_custom_properties#Custom_property_fallback_values

.three {
  background-color: var(--my-var, var(--my-background, pink));
--border-color: linear-gradient(to top, var(--secondary-color), var(--used-color, white));
}