**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`

((frame: any): FrameDev).debugElementStack = [];
              }
              this.stack.push(frame);
              return open;
            }
            invariant(
              false,
              'ReactDOMServer does not yet support the fundamental API.',
            );
          }
          
          case REACT_LAZY_TYPE:
            invariant(
              false,
              'ReactDOMServer does not yet support lazy-loaded components.',
            );
        }
      }
      let info = '';
      if (__DEV__) {
        const owner = nextElement._owner;
        if (
          elementType === undefined ||
          (typeof elementType === 'object' &&
            elementType !== null &&
            Object.keys(elementType).length === 0)
        ) {
          info +=
            ' You likely forgot to export your component from the file ' +
            "it's defined in, or you might have mixed up default and " +