**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`

const didWarnAboutNoopUpdateForComponent = {};
const didWarnAboutBadClass = {};
const didWarnAboutModulePatternComponent = {};
const didWarnAboutDeprecatedWillMount = {};
const didWarnAboutUndefinedDerivedState = {};
const didWarnAboutUninitializedState = {};
const valuePropNames = ['value', 'defaultValue'];
const newlineEatingTags = {
  listing: true,
  pre: true,
  textarea: true,
};
const VALID_TAG_REGEX = /^[a-zA-Z][a-zA-Z:_\.\-\d]*$/; 
const validatedTagCache = {};
function validateDangerousTag(tag) {
  if (!validatedTagCache.hasOwnProperty(tag)) {
    invariant(VALID_TAG_REGEX.test(tag), 'Invalid tag: %s', tag);
    validatedTagCache[tag] = true;
  }
}
const styleNameCache = {};
const processStyleName = function(styleName) {
  if (styleNameCache.hasOwnProperty(styleName)) {
    return styleNameCache[styleName];
  }
  const result = hyphenateStyleName(styleName);
  styleNameCache[styleName] = result;
  return result;
};
function createMarkupForStyles(styles): string | null {