**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`

import lowPriorityWarning from 'shared/lowPriorityWarning';
import warning from 'shared/warning';
import warningWithoutStack from 'shared/warningWithoutStack';
import describeComponentFrame from 'shared/describeComponentFrame';
import ReactSharedInternals from 'shared/ReactSharedInternals';
import {
  warnAboutDeprecatedLifecycles,
  enableSuspenseServerRenderer,
  enableFundamentalAPI,
  enableFlareAPI,
} from 'shared/ReactFeatureFlags';
import {
  REACT_FORWARD_REF_TYPE,
  REACT_FRAGMENT_TYPE,
  REACT_STRICT_MODE_TYPE,
  REACT_CONCURRENT_MODE_TYPE,
  REACT_SUSPENSE_TYPE,
  REACT_SUSPENSE_LIST_TYPE,
  REACT_PORTAL_TYPE,
  REACT_PROFILER_TYPE,
  REACT_PROVIDER_TYPE,
  REACT_CONTEXT_TYPE,
  REACT_LAZY_TYPE,
  REACT_MEMO_TYPE,
  REACT_FUNDAMENTAL_TYPE,
} from 'shared/ReactSymbols';
import {
  emptyObject,
  processContext,
  validateContextBounds,