DarkMode settings apply globally per tenant and not per user

[billal280]

In the lastest version of OpenSearch Dashboards (2.18.0), darkmode settings is user-specific but only applies globally within the "global" tenant. This means that if a user in tenant X activates dark mode, all other users within that same tenant are affected, and vice versa.
[#2159]

Similarly, users must manually switch to dark mode for each tenant they have access to. However, logically, when a user decides to enable dark mode, it should apply across all the tenants they access.

This creates inconsistencies in the user experience when navigating between different tenants.