## Why The app currently ships with a warm light theme only. Adding a system-aware dark mode with a manual toggle removes eye strain in low-light conditions and meets baseline accessibility expectations for a personal finance tool used at any hour. ## What Changes - Add a class-based dark mode variant using Tailwind v4 `@custom-variant` so all `dark:` utilities are controlled by a `dark` class on ``. - Add a `ThemeToggle` component that persists the user's preference in `localStorage` and initialises the correct class before first paint to prevent flash. - Update the root layout to inject the inline initialisation script and render the toggle inside the existing header. - Apply `dark:` variants to all UI components to produce a warm, deep-stone dark palette consistent with the existing amber/stone design language. ## Capabilities ### New Capabilities - `dark-mode`: Users can switch between light and dark themes with a toggle that persists across sessions. System preference is respected on first visit. ### Modified Capabilities - `expense-tracking`: Expense workspace and recurring expense manager now render correctly in both themes. - `paycheck-tracking`: Paycheck workspace now renders correctly in both themes. - `monthly-dashboard`: Dashboard sections, insight cards, category bars, and stat cards now render correctly in both themes. ## Impact - Affected code: `globals.css`, `layout.tsx`, all components under `src/components/`, and page header text in `src/app/`. - APIs: None. - Dependencies: None — uses Tailwind v4 built-ins and the Web Storage API. - Systems: No server-side changes required; theme state is fully client-side.