**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`

false,
          '%s.getChildContext(): childContextTypes must be defined in order to ' +
            'use getChildContext().',
          getComponentName(Component) || 'Unknown',
        );
      }
    }
    if (childContext) {
      context = Object.assign({}, context, childContext);
    }
  }
  return {child, context};
}
type Frame = {
  type: mixed,
  domNamespace: string,
  children: FlatReactChildren,
  fallbackFrame?: Frame,
  childIndex: number,
  context: Object,
  footer: string,
};
type FrameDev = Frame & {
  debugElementStack: Array<ReactElement>,
};
class ReactDOMServerRenderer {
  threadID: ThreadID;