design-systems: add 74 brand DESIGN.md files (VoltAgent/awesome-design-md)

- 74 brands: airbnb, stripe, apple, figma, linear, vercel, etc.
- 147 files: DESIGN.md + README.md per brand (~2.1 MB)
- Each DESIGN.md contains structured brand analysis (colors, typography, components)
- Updated 00-overview.md with new design-systems directory

concepts/dark-mode-design.md

@@ -0,0 +1,57 @@
+---
+title: Dark Mode Design Patterns
+description: Best practices for dark-themed premium websites.
+tags: [design, dark-mode, accessibility, patterns]
+---
+
+# Dark Mode Design Patterns
+
+## Core Principles
+
+### 1. Not Pure Black
+- Background: `#0a0a0a` or `#111111` (not `#000000`)
+- Pure black causes eye strain on OLED
+- Subtle gradient: `#0a0a0a` → `#0f0f0f`
+
+### 2. Surface Hierarchy
+- Layer 1 (bg): `#0a0a0a`
+- Layer 2 (cards): `#111111`
+- Layer 3 (elevated): `#1a1a1a`
+- Elevation via lighter shade, not shadow
+
+### 3. Text Colors
+- Primary: `#f5f5f5` (90% white)
+- Secondary: `#a0a0a0` (60% white)
+- Muted: `#666666` (40% white)
+- Never use pure `#ffffff` (too harsh)
+
+### 4. Border Treatment
+- Use `rgba(255,255,255,0.08)` for subtle borders
+- Hover states: `rgba(255,255,255,0.15)`
+- Focus states: `accent-color` ring
+
+### 5. Accent Colors
+- Lighter variants work better on dark
+- `#6366f1` → `#818cf8` for hover
+- Ensure 3:1 contrast ratio for accents
+
+## CSS Implementation
+```css
+:root {
+  --bg-primary: #0a0a0a;
+  --bg-surface: #111111;
+  --bg-elevated: #1a1a1a;
+  --text-primary: #f5f5f5;
+  --text-secondary: #a0a0a0;
+  --text-muted: #666666;
+  --border-subtle: rgba(255,255,255,0.08);
+  --accent-primary: #6366f1;
+  --accent-hover: #818cf8;
+}
+```
+
+## Anti-Patterns
+- Never use pure `#000000` background
+- Never use pure `#ffffff` text
+- Never rely on shadows for elevation in dark mode
+- Never skip contrast checking (WCAG AA minimum)