feat(web): add theme definition system with 5 built-in themes (#493)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/web/src/themes/dark.ts

@@ -0,0 +1,41 @@
+import type { ThemeDefinition } from "./types";
+
+export const darkTheme: ThemeDefinition = {
+  id: "dark",
+  name: "Dark",
+  description: "Default dark theme — deep navy with vibrant accents",
+  author: "Mosaic Stack",
+  isDark: true,
+  colorPreview: ["#0f141d", "#1b2331", "#eef3ff", "#2f80ff", "#8b5cf6"],
+  colors: {
+    "bg-950": "#080b12",
+    "bg-900": "#0f141d",
+    "bg-850": "#151b26",
+    "surface-800": "#1b2331",
+    "surface-750": "#232d3f",
+    "border-700": "#2f3b52",
+    "text-100": "#eef3ff",
+    "text-300": "#c5d0e6",
+    "text-500": "#8f9db7",
+    "blue-500": "#2f80ff",
+    "blue-400": "#56a0ff",
+    "red-500": "#e5484d",
+    "red-400": "#f06a6f",
+    "purple-500": "#8b5cf6",
+    "purple-400": "#a78bfa",
+    "teal-500": "#14b8a6",
+    "teal-400": "#2dd4bf",
+    "amber-500": "#f59e0b",
+    "amber-400": "#fbbf24",
+    "pink-500": "#ec4899",
+    "emerald-500": "#10b981",
+    "orange-500": "#f97316",
+    "cyan-500": "#06b6d4",
+    "indigo-500": "#6366f1",
+  },
+  shadows: {
+    sm: "0 1px 2px 0 rgb(0 0 0 / 0.3)",
+    md: "0 4px 6px -1px rgb(0 0 0 / 0.4), 0 2px 4px -2px rgb(0 0 0 / 0.3)",
+    lg: "0 10px 15px -3px rgb(0 0 0 / 0.5), 0 4px 6px -4px rgb(0 0 0 / 0.4)",
+  },
+};