postcss-in-styled-components.js

// Basically you put any "dynamic" styles in regular css
// and any postcss as an interpolated variable thing and
// then write a quick babel transform like https://github.com/4Catalyzer/css-literal-loader
// the downside being you can't use props with the postcss stuff

styled.div`
  border-top: ${({top}) => top}; 
  ${postcssify`
  @mixin 'adfsa';
  @whateverFancyThing;
  `}
`