**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`

this.exhausted = true;
          freeThreadID(this.threadID);
          break;
        }
        const frame: Frame = this.stack[this.stack.length - 1];
        if (suspended || frame.childIndex >= frame.children.length) {
          const footer = frame.footer;
          if (footer !== '') {
            this.previousWasTextNode = false;
          }
          this.stack.pop();
          if (frame.type === 'select') {
            this.currentSelectValue = null;
          } else if (
            frame.type != null &&
            frame.type.type != null &&
            frame.type.type.$$typeof === REACT_PROVIDER_TYPE
          ) {
            const provider: ReactProvider<any> = (frame.type: any);
            this.popProvider(provider);
          } else if (frame.type === REACT_SUSPENSE_TYPE) {
            this.suspenseDepth--;
            const buffered = out.pop();
            if (suspended) {
              suspended = false;
              
              const fallbackFrame = frame.fallbackFrame;
              invariant(
                fallbackFrame,
                'suspense fallback not found, something is broken',