Create 'Fiori theme' related E2E App tests

[UlianaMunich]

Create a theme switch somewhere in top nav (maybe in profile menu?) to easily switch from light to dark mode so that theming related issues can be debugged/tested easily.

Example as inspiration: https://github.tools.sap/ui/btp-demo-luigi/blob/a191d5cc10242438435f8a44591d2f8c160fb58f/main/src/config.js#L110 (mutationObserver can be ignored and theme should come from local theming-base-content instead of cdn)