[Look & Feel] Add dark mode option that respects browser settings

[joshuarrrr]

Administrators should have to option to configure OpenSearch Dashboards to respect individual user browser settings for light mode/dark mode.

Currently, switching between light and dark mode theme variants can only be done via the Advanced Settings. Whichever mode is selected affects all users of that instance, regardless of their own preferences or system settings. We'd like to add a third option to that setting "Respect user browser settings", which would allow user preferences to override the default light/dark behavior.

Ideally, if users have their system or browser configured to switch between light and dark mode at various times of day, the application would adjust automatically. But due to the current implementation, we'll start with the requirement that at page load/refresh, the application gets dark/light mode preferences from the browser and uses the preferred theme accordingly.

In absence of any user/browser preferences, dark mode will be used by default.

Related:
#4454