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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));
}

All in the title :) This should be documented, as we can put a lot of stuffs unrelated to colors in a theme, and I am not sure that the library will skip the non color props (I'll try asap).

Now that custom properties have been added to the switch module, I should add a section to the contributing guide, regarding the intended use & strategy for custom props going forward,

Try to put background image as a last style in a rule but without a semicolon delimiter. This is valid CSS. The last semicolon is optional in the last line of the CSS rule. But this brakes building with react-scoped-styles. The issue doesn't apear if semicolon is always present. Can't be reproduced if the last line doesn't contain url(...) function.

![image](https://user-images.githubuserco