[User profile] Add support for system theme

[pgayvallet]

#89340 is a feature request to have a way for Kibana to select its theme based on the system's preference (system theme)

#173044 implements it by introducing a third option for the theme:darkMode uiSetting, and adding all the implementation details for this new option to be properly used.

However, this PR only adds the feature at the uiSettings level, introducing a feature gap between the two systems (uiSettings/advanced settings vs user profile).

This issue is here to address this feature gap, by adding support for this system theme option to the user profile.

From my understanding, given that #173044 already implements all the internal implementation of the theme switch / system theme detection, it should be rather straightforward (only adapting the user profile UI and the user settings service that was introduced in #151507), but I'd like the security team to confirm that point.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[legrego]

Awaiting #173044

[pgayvallet]

FYI, #173044 has been merged 🎉

[legrego]

Closing as duplicate of #155945