**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`

didWarnAboutUninitializedState[componentName] = true;
            }
          }
        }
        let partialState = Component.getDerivedStateFromProps.call(
          null,
          element.props,
          inst.state,
        );
        if (__DEV__) {
          if (partialState === undefined) {
            const componentName = getComponentName(Component) || 'Unknown';
            if (!didWarnAboutUndefinedDerivedState[componentName]) {
              warningWithoutStack(
                false,
                '%s.getDerivedStateFromProps(): A valid state object (or null) must be returned. ' +
                  'You have returned undefined.',
                componentName,
              );
              didWarnAboutUndefinedDerivedState[componentName] = true;
            }
          }
        }
        if (partialState != null) {
          inst.state = Object.assign({}, inst.state, partialState);
        }
      }
    } else {
      if (__DEV__) {
        if (