**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 (!nextStyles) {
    return;
  }
  const expandedUpdates = expandShorthandMap(styleUpdates);
  const expandedStyles = expandShorthandMap(nextStyles);
  const warnedAbout = {};
  for (const key in expandedUpdates) {
    const originalKey = expandedUpdates[key];
    const correctOriginalKey = expandedStyles[key];
    if (correctOriginalKey && originalKey !== correctOriginalKey) {
      const warningKey = originalKey + ',' + correctOriginalKey;
      if (warnedAbout[warningKey]) {
        continue;
      }
      warnedAbout[warningKey] = true;
      warning(
        false,
        '%s a style property during rerender (%s) when a ' +
          'conflicting property is set (%s) can lead to styling bugs. To ' +
          "avoid this, don't mix shorthand and non-shorthand properties " +
          'for the same value; instead, replace the shorthand with ' +
          'separate values.',
        isValueEmpty(styleUpdates[originalKey]) ? 'Removing' : 'Updating',
        originalKey,
        correctOriginalKey,
      );
    }
  }
}