-webkit-appearance is not supported by theme-ui sx property

[joshball]

The code in feed-search.js (the reference to -webkit-appearance):

       <Box sx={{ flexGrow: 1 }}>
          <input
            type="search"
            aria-label="search"
            sx={{
              '-webkit-appearance': 'textfield',
              width: ['100%'],
              ...

Is causing theme-ui to throw an error:

Using kebab-case for css properties in objects is not supported. Did you mean WebkitAppearance?

Should that be pulled out to a local style? Global style? Not sure the best approach here.