**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`

ReactControlledValuePropTypes.checkPropTypes('select', props);
        for (let i = 0; i < valuePropNames.length; i++) {
          const propName = valuePropNames[i];
          if (props[propName] == null) {
            continue;
          }
          const isArray = Array.isArray(props[propName]);
          if (props.multiple && !isArray) {
            warning(
              false,
              'The `%s` prop supplied to <select> must be an array if ' +
                '`multiple` is true.',
              propName,
            );
          } else if (!props.multiple && isArray) {
            warning(
              false,
              'The `%s` prop supplied to <select> must be a scalar ' +
                'value if `multiple` is false.',
              propName,