**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`

currentDebugStacks.push(stack);
    if (currentDebugStacks.length === 1) {
      
      
      prevGetCurrentStackImpl = ReactDebugCurrentFrame.getCurrentStack;
      ReactDebugCurrentFrame.getCurrentStack = getCurrentServerStackImpl;
    }
  };
  pushElementToDebugStack = function(element: ReactElement) {
    
    const stack = currentDebugStacks[currentDebugStacks.length - 1];
    
    const frame: Frame = stack[stack.length - 1];
    
    ((frame: any): FrameDev).debugElementStack.push(element);
    
    
    
  };
  popCurrentDebugStack = function() {
    currentDebugStacks.pop();
    if (currentDebugStacks.length === 0) {
      
      
      ReactDebugCurrentFrame.getCurrentStack = prevGetCurrentStackImpl;
      prevGetCurrentStackImpl = null;
    }
  };
  getCurrentServerStackImpl = function(): string {
    if (currentDebugStacks.length === 0) {