Theme-ui: Theme color for caretColor on input element not resolved properly

[henrikland]

Describe the bug
Setting the caretColor prop on an input element to a color defined in theme.colors renders broken CSS.

To Reproduce
Steps to reproduce the behavior:

1. Define some color in your theme, for example accent: "#FF0000".
2. Set the caretColor prop to "accent" on an input:
   <input sx={{ caretColor: "accent" }}>...</input>
3. Rendered CSS will be caret-color: accent.

Expected behavior
Expected rendered CSS to be caret-color: #FF0000.

[jxnblk]

This property is not currently mapped to the theme.colors scale -- in the meantime, you can use functional values

[lachlanjc]

https://github.com/system-ui/theme-ui/pull/780 will close this issue :)