Include information about the dark mode setting of the web interface

[schmanu]

What problem does it solve?

Currently safe-apps cannot retrieve the information if the web interface has the dark mode enabled.
Therefore many safe apps only offer a light mode which does not look consistent with the web interface if dark mode is enabled.

How do we solve it?

We should add the information if the dark mode is enabled or not into the data the safe-apps-sdk retrieves from the web-core communicator. We could create a new data structure "settings" which for now only holds the info if the dark mode is enabled.

Then safe apps could react to that setting and change the theme accordingly.

[katspaugh]

@dasanra @mikhailxyz do you mind if we tackle this? @schmanu is eager.