This function doesn't have a /*#__PURE__*/ prefix, which breaks tree-shaking. Moreover, when I try to prepend it myself through inject option it gets stripped along with the entire string from the final bundle, which leaves you without CSS.

function inject(cssVariableName) {
  return (
    '\n' +
    `/*#__PURE__*/ (${styleInjectFork})(${cssVariableName}${
      Object.keys(styleInjectOptions).length > 0
        ? `,${JSON.stringify(styleInjectOptions, function(_key, value) {
            if (typeof value === 'function') {
              return value.toString();
            }
            return value;
          })}`
        : ''
    });`
  );
}

In my case, I forked styleInject, because it's missing some features, so I had to compose it slightly different into the string (wrap it with brackets to turn it into expression, which is then invokable).