Theme-ui: [typescript] Sx properties should accept booleans

[Ericnr]

It's very important that the sx properties are typed so that you can write sx={{ color: isActive && 'blue' }}. It does behaves correctly when a boolean is passed, the property is simply ignored, it's only a typescript since it expects only strings.

[hasparus]

Fair point. We should accept false in place of null and undefined for convenience.