[feature] Dark mode

As a User

I want to choose from Light or dark UI mode

So that I can set up the UI according to my preferences

Acceptance Criteria
TODO

The issue names no files, tests, or entry points, and its acceptance criteria are still TODO. Start by mapping the UI entry points and styling or theme configuration, then clarify the required mode-selection and preference behavior. Done should be defined by acceptance criteria covering the intended interface and persistence scope.

Written by the indexing model from the issue text.