**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`

if (child === undefined) {
    invariant(
      false,
      '%s(...): Nothing was returned from render. This usually means a ' +
        'return statement is missing. Or, to render nothing, ' +
        'return null.',
      getComponentName(type) || 'Component',
    );
  }
}
function resolve(
  child: mixed,
  context: Object,
  threadID: ThreadID,
): {|
  child: mixed,
  context: Object,
|} {
  while (React.isValidElement(child)) {