style.js

// Prop receivers
const borderProps = props => css`
  ${props.borderBottom && `border-bottom: ${props.borderWidth || "1px"} solid ${color.border}`};
  ${props.borderTop && `border-top: ${props.borderWidth || "1px"} solid ${color.border}`};
  ${props.borderLeft && `border-left: ${props.borderWidth || "1px"} solid ${color.border}`};
  ${props.borderRight && `border-right: ${props.borderWidth || "1px"} solid ${color.border}`};
`;

// Mixins
const size = (width, height = width) => `
  width: ${width};
  height: ${height};
`;

// Example
const Button = styled.button`
  ${borderProps};
  ${size('200px', '50px')};
`;