**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`

popCurrentDebugStack();
          }
        }
        if (out.length <= this.suspenseDepth) {
          out.push('');
        }
        out[this.suspenseDepth] += outBuffer;
      }
      return out[0];
    } finally {
      ReactCurrentDispatcher.current = prevDispatcher;
      setCurrentThreadID(prevThreadID);
    }
  }
  render(
    child: ReactNode | null,
    context: Object,
    parentNamespace: string,
  ): string {
    if (typeof child === 'string' || typeof child === 'number') {
      const text = '' + child;
      if (text === '') {
        return '';
      }
      if (this.makeStaticMarkup) {
        return escapeTextForBrowser(text);
      }
      if (this.previousWasTextNode) {
        return '<!-- -->' + escapeTextForBrowser(text);
      }