[Config] Dark mode config option + persistent store about dietpi-dashboard

[MichaIng]

However, is this really worth a setting in the backend?

Maybe you're right. When we add color themes as well, I thought that allowing to set a default for new clients and when clearing browser data would be nice. But at least for now having it persistent in browser storage should be fine. We'll see whether a config file setting is required by others or not.

[ravenclaw900]

Indeed, it could be passed with update and login in the "global" section. However, is this really worth a setting in the backend? It seems easier to just keep the dark mode setting in localStorage on the frontend, depending on what the user selected with the button.