**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 componentName = getComponentName(Component) || 'Unknown';
        if (!didWarnAboutModulePatternComponent[componentName]) {
          warningWithoutStack(
            false,
            'The <%s /> component appears to be a function component that returns a class instance. ' +
              'Change %s to a class that extends React.Component instead. ' +
              "If you can't use a class try assigning the prototype on the function as a workaround. " +
              "`%s.prototype = React.Component.prototype`. Don't use an arrow function since it " +
              'cannot be called with `new` by React.',
            componentName,
            componentName,
            componentName,
          );
          didWarnAboutModulePatternComponent[componentName] = true;
        }
      }
    }
    inst.props = element.props;
    inst.context = publicContext;
    inst.updater = updater;
    let initialState = inst.state;
    if (initialState === undefined) {
      inst.state = initialState = null;
    }
    if (
      typeof inst.UNSAFE_componentWillMount === 'function' ||
      typeof inst.componentWillMount === 'function'
    ) {
      if (typeof inst.componentWillMount === 'function') {
        if (__DEV__) {