**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`

let serialized = '';
  let delimiter = '';
  for (const styleName in styles) {
    if (!styles.hasOwnProperty(styleName)) {
      continue;
    }
    const isCustomProperty = styleName.indexOf('--') === 0;
    const styleValue = styles[styleName];
    if (__DEV__) {
      if (!isCustomProperty) {
        warnValidStyle(styleName, styleValue);
      }
    }
    if (styleValue != null) {
      serialized += delimiter + processStyleName(styleName) + ':';
      serialized += dangerousStyleValue(
        styleName,
        styleValue,
        isCustomProperty,
      );
      delimiter = ';';
    }
  }
  return serialized || null;
}
function warnNoop(