[Lens] fix opening formula panel causes "Uncaught Error: Maximum upda… (

#161886)

…te depth exceeded"

## Summary

Fixes #161885. The infinite
update loop doesn't happen as we stabilized the `defaultTheme` object.

src/plugins/kibana_react/public/theme/use_theme.ts

@@ -11,9 +11,9 @@ import useObservable from 'react-use/lib/useObservable';
 import { of } from 'rxjs';
 import { useKibana } from '../context/context';
 
-export const useKibanaTheme = (): CoreTheme => {
-  const defaultTheme: CoreTheme = { darkMode: false };
+const defaultTheme: CoreTheme = { darkMode: false };
 
+export const useKibanaTheme = (): CoreTheme => {
   const {
     services: { theme },
   } = useKibana();