**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`

} from './ReactPartialRendererContext';
import {allocThreadID, freeThreadID} from './ReactThreadIDAllocator';
import {
  createMarkupForCustomAttribute,
  createMarkupForProperty,
  createMarkupForRoot,
} from './DOMMarkupOperations';
import escapeTextForBrowser from './escapeTextForBrowser';
import {
  prepareToUseHooks,
  finishHooks,
  Dispatcher,
  currentThreadID,
  setCurrentThreadID,
} from './ReactPartialRendererHooks';
import {
  Namespaces,
  getIntrinsicNamespace,
  getChildNamespace,
} from '../shared/DOMNamespaces';
import ReactControlledValuePropTypes from '../shared/ReactControlledValuePropTypes';
import assertValidProps from '../shared/assertValidProps';
import dangerousStyleValue from '../shared/dangerousStyleValue';
import hyphenateStyleName from '../shared/hyphenateStyleName';
import isCustomComponent from '../shared/isCustomComponent';
import omittedCloseTags from '../shared/omittedCloseTags';
import warnValidStyle from '../shared/warnValidStyle';
import {validateProperties as validateARIAProperties} from '../shared/ReactDOMInvalidARIAHook';
import {validateProperties as validateInputProperties} from '../shared/ReactDOMNullInputValuePropHook';
import {validateProperties as validateUnknownProperties} from '../shared/ReactDOMUnknownPropertyHook';