**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`

footer: '',
          };
          if (__DEV__) {
            ((frame: any): FrameDev).debugElementStack = [];
          }
          this.stack.push(frame);
          return '';
        }
        case REACT_SUSPENSE_TYPE: {
          if (enableSuspenseServerRenderer) {
            const fallback = ((nextChild: any): ReactElement).props.fallback;
            if (fallback === undefined) {
              
              const nextChildren = toArray(
                ((nextChild: any): ReactElement).props.children,
              );
              const frame: Frame = {
                type: null,
                domNamespace: parentNamespace,
                children: nextChildren,
                childIndex: 0,
                context: context,
                footer: '',
              };
              if (__DEV__) {
                ((frame: any): FrameDev).debugElementStack = [];
              }
              this.stack.push(frame);
              return '';
            }