Debugging CSS can be a headache, especially if you are working on someone else’s code or even your own code written a while back! Contracts in vanilla-extract have you back here. They can be used to make your CSS more robust and maintainable. From the outset, you can stipulate that themes must contain `primaryColour` as an example. Now, because under the hood vanilla-extract uses TypeScript tooling, any new themes missing `primaryColour` will get flagged up as an error in the editor. This also goes for if you add a new, ad-hoc variable to the dark theme without updating the underlying contract. Hopefully you will be setting red borders on elements much less often going forward .