**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`

if (__DEV__) {
    let serialized = '';
    let delimiter = '';
    for (const styleName in styles) {
      if (!styles.hasOwnProperty(styleName)) {
        continue;
      }
      const styleValue = styles[styleName];
      if (styleValue != null) {
        const isCustomProperty = styleName.indexOf('--') === 0;
        serialized += delimiter + hyphenateStyleName(styleName) + ':';
        serialized += dangerousStyleValue(
          styleName,
          styleValue,
          isCustomProperty,
        );
        delimiter = ';';
      }
    }
    return serialized || null;
  }
}
/**
 * Sets the value for multiple styles on a node.  If a value is specified as
 * '' (empty string), the corresponding style property will be unset.
 *
 * @param {DOMElement} node
 * @param {object} styles
 */
export function setValueForStyles(node, styles) {