Checkbox component is broken with styled-components v5x.x

Hi,

We've upgraded our project from styled-components v4 to v5 and we've noticed that the rebass Checkbox form is now broken: the checkbox icon doesn't render when the checkbox is checked, it only renders properly when is disabled.

Here is a repro: with styled-components v4 (everything works fine) https://codesandbox.io/s/elastic-water-h1rg6

with styled-components v5 (Checkbox broken) https://codesandbox.io/s/gallant-heyrovsky-cxbc0

I've taken a look to the styled-components changelog but haven't found anything change that I could associate with the described problem.

PS: thank you for rebass, it's an amazing library :)