**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`

/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */
import {shorthandToLonghand} from './CSSShorthandProperty';
import dangerousStyleValue from './dangerousStyleValue';
import hyphenateStyleName from './hyphenateStyleName';
import warnValidStyle from './warnValidStyle';
import warning from 'shared/warning';
import {warnAboutShorthandPropertyCollision} from 'shared/ReactFeatureFlags';
/**
 * Operations for dealing with CSS properties.
 */
/**
 * This creates a string that is expected to be equivalent to the style
 * attribute generated by server-side rendering. It by-passes warnings and
 * security checks so it's not safe to use this value for anything other than
 * comparison. It is only used in DEV for SSR validation.
 */
export function createDangerousStringForStyles(styles) {