**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`

warning(
          tag === element.type,
          '<%s /> is using incorrect casing. ' +
            'Use PascalCase for React components, ' +
            'or lowercase for HTML elements.',
          element.type,
        );
      }
    }
    validateDangerousTag(tag);
    let props = element.props;
    if (tag === 'input') {
      if (__DEV__) {
        ReactControlledValuePropTypes.checkPropTypes('input', props);
        if (
          props.checked !== undefined &&
          props.defaultChecked !== undefined &&
          !didWarnDefaultChecked
        ) {
          warning(
            false,
            '%s contains an input of type %s with both checked and defaultChecked props. ' +
              'Input elements must be either controlled or uncontrolled ' +
              '(specify either the checked prop, or the defaultChecked prop, but not ' +
              'both). Decide between using a controlled or uncontrolled input ' +
              'element and remove one of these props. More info: ' +
              'https://fb.me/react-controlled-components',
            'A component',
            props.type,