I’ve tried
<TextAreaField
name="themeJSON"
height="70vh"
defaultValue={props.theme?.themeJSON}
className="rw-input"
errorClassName="rw-input rw-input-error"
validation={{ required: true }}
/>
and
<TextAreaField
name="themeJSON"
sx={{ height: '70vh' }}
defaultValue={props.theme?.themeJSON}
className="rw-input"
errorClassName="rw-input rw-input-error"
validation={{ required: true }}
/>