diff --git a/docs/design_specification.md b/docs/design_specification.md
new file mode 100644
index 0000000..3bf49db
--- /dev/null
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@@ -0,0 +1,198 @@
+# Fulling Design Specification
+
+This document defines the visual design standards for the Fulling project. All pages and components must adhere to this specification.
+
+---
+
+## Design Philosophy
+
+### Core Principles
+
+1. **VS Code-Inspired Dark Theme** — Familiar interface for developers
+2. **Two-Section Sidebar Architecture** — Visual separation between navigation and project areas
+3. **Semantic Naming** — CSS variables named by purpose for easy maintenance
+4. **WCAG Accessibility** — Contrast ratios meet accessibility standards
+
+### Visual Hierarchy
+
+| Level | Background | Usage |
+|-------|------------|-------|
+| Main Background | `#1E1E1E` | Editor/content area |
+| Panel Background | `#252526` | Cards, modals, tab containers |
+| Sidebar Navigation | `#333333` | Primary navigation area |
+| Sidebar Project | `#242426` | Project list area |
+
+---
+
+## Color System
+
+### Primary Colors
+
+| Variable | Value | Usage |
+|----------|-------|-------|
+| `--primary` | `#10639D` | Brand color (buttons, links, accents) |
+| `--primary-hover` | `#155a8a` | Primary hover state |
+| `--primary-foreground` | `#FFFFFF` | Text on primary elements |
+
+### Functional Colors
+
+| Variable | Usage |
+|----------|-------|
+| `--destructive` | Delete actions, error states (red spectrum) |
+| `--accent` | Hover states, highlights (`#2B2D2E`) |
+| `--muted` | Supporting element backgrounds |
+| `--muted-foreground` | Secondary text (`#808080`) |
+
+### Text Colors
+
+| Variable | Value | Usage |
+|----------|-------|-------|
+| `--foreground` | `#CCCCCC` | Primary text |
+| `--muted-foreground` | `oklch(0.72 0 0)` | Secondary text, descriptions |
+| `--card-foreground` | `#FFFFFF` | Card titles only |
+
+### Borders
+
+| Variable | Value | Usage |
+|----------|-------|-------|
+| `--border` | `#3E3E3E` | General borders |
+| `--input` | `oklch(0.37 0 0)` | Input field borders |
+| `--ring` | `oklch(0.62 0.19 259.76)` | Focus ring |
+
+---
+
+## Theme Configuration
+
+The project supports two dark themes, switched via CSS class:
+
+### `.dark` (Default)
+- Primary: `#10639D` (deep blue)
+- Suitable for: General use
+
+### `.stealth` (Alternate)
+- Primary: `#007acc` (VS Code blue)
+- Suitable for: Pure VS Code experience
+
+> **Note**: Light theme (`:root`) definitions exist but are incomplete. Do not use.
+
+---
+
+## Typography
+
+| Variable | Font | Usage |
+|----------|------|-------|
+| `--font-mono` | JetBrains Mono, monospace | Code, terminal |
+| `--font-serif` | Source Serif 4, serif | Headings, emphasis |
+
+---
+
+## Border Radius System
+
+| Variable | Computed Value | Usage |
+|----------|---------------|-------|
+| `--radius` | `0.375rem` | Base radius |
+| `--radius-sm` | `calc(var(--radius) - 4px)` | Small elements |
+| `--radius-md` | `calc(var(--radius) - 2px)` | Medium elements |
+| `--radius-lg` | `var(--radius)` | Large elements |
+| `--radius-xl` | `calc(var(--radius) + 4px)` | Cards, modals |
+
+---
+
+## Shadow System
+
+Multi-level shadows optimized for dark theme:
+
+| Variable | Usage |
+|----------|-------|
+| `--shadow-xs` | Minimal shadow |
+| `--shadow-sm` | Small shadow (default buttons) |
+| `--shadow-md` | Medium shadow |
+| `--shadow-lg` | Large shadow (modals) |
+| `--shadow-xl` | Extra large shadow |
+
+---
+
+## Component Specifications
+
+### Button
+
+**Variants:**
+
+| Variant | Style | Usage |
+|---------|-------|-------|
+| `default` | Primary background + white text | Primary actions |
+| `secondary` | Secondary background + border | Secondary actions |
+| `outline` | Transparent + border | Neutral actions |
+| `ghost` | Transparent | Toolbar buttons |
+| `destructive` | Red background | Dangerous actions |
+| `link` | Link style | Text links |
+
+**Sizes:**
+
+| Size | Height | Usage |
+|------|--------|-------|
+| `sm` | `h-8` | Compact contexts |
+| `default` | `h-9` | Standard buttons |
+| `lg` | `h-10` | Emphasized buttons |
+| `icon` | `size-9` | Icon buttons |
+| `icon-sm` | `size-8` | Small icon buttons |
+| `icon-lg` | `size-10` | Large icon buttons |
+
+### Card
+
+- Background: `bg-card`
+- Border Radius: `rounded-xl`
+- Padding: `py-6 px-6`
+- Shadow: `shadow-sm`
+- Border: `border`
+
+### Tabs
+
+| State | Background | Text |
+|-------|------------|------|
+| Container | `#252526` | — |
+| Inactive | `#292929` | `#A0A0A0` |
+| Active | `#1E1E1E` | `#FFFFFF` |
+| Hover | `#363637` | — |
+
+---
+
+## Sidebar Specifications
+
+### Primary Navigation
+
+| Variable | Value |
+|----------|-------|
+| `--sidebar-background` | `#333333` |
+| `--sidebar-primary-foreground` | `#808080` |
+| `--sidebar-primary-active` | `#FFFFFF` |
+| `--sidebar-primary-hover` | `#2B2D2E` |
+
+### Project Section
+
+| Variable | Value |
+|----------|-------|
+| `--sidebar-project-background` | `#242426` |
+| `--sidebar-project-foreground` | `#CCCCCC` |
+| `--sidebar-project-active-background` | `#3A3D41` |
+
+---
+
+## Chart Colors
+
+Sequential color palette for data visualization (blue-purple spectrum):
+
+| Variable | Usage |
+|----------|-------|
+| `--chart-1` | Primary chart color |
+| `--chart-2` | Secondary chart color |
+| `--chart-3` | Tertiary chart color |
+| `--chart-4` | Quaternary chart color |
+| `--chart-5` | Quinary chart color |
+
+---
+
+## Related Files
+
+- [globals.css](file:///Users/che/Documents/GitHub/fulling/app/globals.css) — CSS variable definitions
+- [style.md](file:///Users/che/Documents/GitHub/fulling/docs/style.md) — Detailed color reference
diff --git a/sandbox/Dockerfile b/sandbox/Dockerfile
index b60295a..316474b 100644
--- a/sandbox/Dockerfile
+++ b/sandbox/Dockerfile
@@ -19,6 +19,7 @@ LABEL maintainer="FullstackAgent" \
 #       This is safe - no actual secrets are hardcoded in the Dockerfile.
 ENV DEBIAN_FRONTEND=noninteractive \
     NODE_VERSION=22.x \
+    NEXT_VERSION=15.5.9 \
     CLAUDE_CODE_VERSION=latest \
     PATH="/root/.local/bin:/home/fulling/.local/bin:$PATH" \
     TERM=xterm-256color \
@@ -82,7 +83,7 @@ RUN set -eux; \
 # -----------------------------------------------------------------------------
 RUN npm install -g \
         @anthropic-ai/claude-code \
-        create-next-app \
+        create-next-app@${NEXT_VERSION} \
         pnpm \
         prisma \
         typescript \
@@ -223,7 +224,7 @@ RUN set -eux; \
     mkdir -p "$TEMPLATE_DIR"; \
     cd "$TEMPLATE_DIR"; \
     echo "=== Creating Next.js project in $TEMPLATE_DIR ==="; \
-    npx --yes create-next-app@latest . \
+    npx --yes create-next-app@${NEXT_VERSION} . \
         --typescript \
         --tailwind \
         --eslint \