**Do you want to request a *feature* or report a *bug*?**

Bug

### Current Behaviour

 - `ReactDOMServer.renderToString` is mutating CSS Variable names that are inlined and in the `--camelCase` format into `--kebab-case`

### Expected Behaviour

 - `ReactDOMServer.renderToString` does not mutate the CSS Variable name, and keeps it in `--camelCase` format

### Demo

 - https://codesandbox.io/s/react-dom-server-debugging-28xsr

### Versions:

 - Issue appears in `react@16.8.6` &  `react-dom@16.8.6` 
 - Issue appears in `react@16.0.0` & `react-dom@16.0.0` 
 - (That's all I've tested so far)

### Other notes:

 - It's worth noting that `ReactDOM.render` is preserving the CSS Variable name and not mutating it. It only seems to only be an issue with `ReactDOMServer.renderToString`

}
        }
        
        default:
          break;
      }
      if (typeof elementType === 'object' && elementType !== null) {
        switch (elementType.$$typeof) {
          case REACT_FORWARD_REF_TYPE: {
            const element: ReactElement = ((nextChild: any): ReactElement);
            let nextChildren;
            const componentIdentity = {};
            prepareToUseHooks(componentIdentity);
            nextChildren = elementType.render(element.props, element.ref);
            nextChildren = finishHooks(
              elementType.render,
              element.props,
              nextChildren,
              element.ref,
            );
            nextChildren = toArray(nextChildren);
            const frame: Frame = {
              type: null,
              domNamespace: parentNamespace,
              children: nextChildren,
              childIndex: 0,
              context: context,
              footer: '',
            };
            if (__DEV__) {