[SB] Fix graph layout stories (elastic#217766)

## Summary

This fixes the graph layout stories not rendering properly after
elastic#214684. Open to a better or preferred fix as I'm not familiar with
these components.

There was a console warning:
```js
[React Flow]: The React Flow parent container needs a width and a height to render the graph. Help: https://reactflow.dev/error#004
```

[Storybook preview
link](https://ci-artifacts.kibana.dev/storybooks/pr-217766/cloud_security_posture_graph/index.html?path=/story/components-graph-components-graph-layout--simple-api-mock)

---------

Co-authored-by: Kfir Peled <[email protected]>

Author: 2 people

x-pack/solutions/security/packages/kbn-cloud-security-posture/graph/.storybook/decorators/global_styles_decorator.tsx

@@ -10,7 +10,7 @@ import { Global, css } from '@emotion/react';
 const globalStyles = css`
   html,
   body,
-  #root {
+  #storybook-root {
     width: 100%;
     height: 100%;
     margin: 0;