refactor: use CSS-var-with-fallback for dark theme colors in web/

Allows platform-specific dark color overrides (e.g. Windows Electron)
while keeping identical fallback values for web/ (no visual change).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/tailwind.config.ts

@@ -9,9 +9,9 @@ export default {
         primary: "rgb(var(--primary) / <alpha-value>)",
         "primary-hover": "rgb(var(--primary-hover) / <alpha-value>)",
         "bg-light": "#f6f8f6",
-        "bg-dark": "#0d1117",
-        "card-dark": "#161b22",
-        "border-dark": "#30363d",
+        "bg-dark": "var(--bg-dark, #0d1117)",
+        "card-dark": "var(--card-dark, #161b22)",
+        "border-dark": "var(--border-dark, #30363d)",
         "text-main": "#e6edf3",
         "text-dim": "rgb(139 148 158 / <alpha-value>)",
       },