**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 (
            warnAboutDeprecatedLifecycles &&
            inst.componentWillMount.__suppressDeprecationWarning !== true
          ) {
            const componentName = getComponentName(Component) || 'Unknown';
            if (!didWarnAboutDeprecatedWillMount[componentName]) {
              lowPriorityWarning(
                false,
                
                'componentWillMount has been renamed, and is not recommended for use. ' +
                  'See https://fb.me/react-async-component-lifecycle-hooks for details.\n\n' +
                  '* Move code from componentWillMount to componentDidMount (preferred in most cases) ' +
                  'or the constructor.\n' +
                  '\nPlease update the following components: %s',
                componentName,
              );
              didWarnAboutDeprecatedWillMount[componentName] = true;
            }
          }
        }