**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`

return value == null || typeof value === 'boolean' || value === '';
}
/**
 * Given {color: 'red', overflow: 'hidden'} returns {
 *   color: 'color',
 *   overflowX: 'overflow',
 *   overflowY: 'overflow',
 * }. This can be read as "the overflowY property was set by the overflow
 * shorthand". That is, the values are the property that each was derived from.
 */
function expandShorthandMap(styles) {