Act as a senior front-end developer with 5+ years of experience in responsive design. Create a step-by-step guide for implementing a dark mode toggle using only CSS (no JavaScript). Include: 1. A [COLOR_SCHEME] media query to detect system preferences 2. CSS variables for [LIGHT_THEME_COLORS] and [DARK_THEME_COLORS] 3. A toggle switch styled with pure CSS that persists the user's preference using the :checked pseudo-class 4. Smooth transitions between themes using [TRANSITION_DURATION] Provide the complete CSS code with comments explaining each section. Focus on accessibility (contrast ratios) and fallbacks for older browsers.