**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`

type ReactNode = string | number | ReactElement;
type FlatReactChildren = Array<null | ReactNode>;
type toArrayType = (children: mixed) => FlatReactChildren;
const toArray = ((React.Children.toArray: any): toArrayType);
let currentDebugStacks = [];
let ReactCurrentDispatcher = ReactSharedInternals.ReactCurrentDispatcher;
let ReactDebugCurrentFrame;
let prevGetCurrentStackImpl = null;
let getCurrentServerStackImpl = () => '';
let describeStackFrame = element => '';
let validatePropertiesInDevelopment = (type, props) => {};
let pushCurrentDebugStack = (stack: Array<Frame>) => {};
let pushElementToDebugStack = (element: ReactElement) => {};
let popCurrentDebugStack = () => {};
let hasWarnedAboutUsingContextAsConsumer = false;
if (__DEV__) {
  ReactDebugCurrentFrame = ReactSharedInternals.ReactDebugCurrentFrame;
  validatePropertiesInDevelopment = function(type, props) {
    validateARIAProperties(type, props);
    validateInputProperties(type, props);
    validateUnknownProperties(type, props,  false);
  };
  describeStackFrame = function(element): string {
    const source = element._source;
    const type = element.type;
    const name = getComponentName(type);
    const ownerName = null;
    return describeComponentFrame(name, source, ownerName);
  };
  pushCurrentDebugStack = function(stack: Array<Frame>) {