Add dark mode to my {{tech_stack}} site. Our brand colors are {{brand_colors}}. Read the existing styles first and show me a short plan before changing files. Requirements: - Three choices, Light, Dark and System, defaulting to System; the header toggle has an accessible name and shows the current choice - Remember the choice on the device and apply it before the first paint, so the wrong theme never flashes on load - Semantic color tokens, such as background, surface, text, muted text, border and accent, instead of separate styles per theme - A designed dark palette, not an inversion: very dark gray rather than pure black, softened brand colors, and accents lightened enough to stand out - Text contrast of at least 4.5:1, and 3:1 for icons, borders and focus rings, in both themes, meeting WCAG 2.2 AA - The page's color scheme set so form fields and scrollbars match, and the browser theme color updated - Dark-friendly versions of logos and illustrations, a subtle border on screenshots, and lighter surfaces instead of heavy shadows - Every page and state checked in both themes: hover, focus, errors, empty states, dialogs, charts and embeds When you're done, list the files changed, any colors that failed contrast and how you fixed them, and anything you assumed.