**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`

Component.prototype &&
          typeof Component.prototype.render === 'function'
        ) {
          const componentName = getComponentName(Component) || 'Unknown';
          if (!didWarnAboutBadClass[componentName]) {
            warningWithoutStack(
              false,
              "The <%s /> component appears to have a render method, but doesn't extend React.Component. " +
                'This is likely to cause errors. Change %s to extend React.Component instead.',
              componentName,
              componentName,
            );
            didWarnAboutBadClass[componentName] = true;
          }
        }
      }
      const componentIdentity = {};
      prepareToUseHooks(componentIdentity);
      inst = Component(element.props, publicContext, updater);
      inst = finishHooks(Component, element.props, inst, publicContext);
      if (inst == null || inst.render == null) {
        child = inst;
        validateRenderResult(child, Component);
        return;
      }
      if (__DEV__) {