**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`

child = null;
      }
    }
    validateRenderResult(child, Component);
    let childContext;
    if (typeof inst.getChildContext === 'function') {
      let childContextTypes = Component.childContextTypes;
      if (typeof childContextTypes === 'object') {
        childContext = inst.getChildContext();
        for (let contextKey in childContext) {
          invariant(
            contextKey in childContextTypes,
            '%s.getChildContext(): key "%s" is not defined in childContextTypes.',
            getComponentName(Component) || 'Unknown',
            contextKey,
          );
        }
      } else {
        warningWithoutStack(