Interpolation in Cssta is really a work in progress. It will never support interpolating functions. It will eventually support mixins and handling keyframes like styled-components. However, for now, you can only interpolate single values.
const Component = cssta(View)`
border-bottom: ${StyleSheet.hairlineWidth}px solid grey;
`;
const Component = cssta(View)`
${rules};
border: ${props => props.color};
`;