Add dark theme + auto accent color overrides

Use off-white backgrounds with dark text for active/selected elements
when using dark theme with 'auto' accent color:
- Tab buttons (active state)
- Sidebar nav items (active state)
- Primary buttons
- Toggle switches
- Badges, checkboxes, input focus states
- Theme settings cards and options

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/styles/Sidebar.css

@@ -1018,4 +1018,18 @@
     transform-origin: bottom center;
     animation: slideUpFade 0.25s cubic-bezier(0.4, 0, 0.2, 1);
   }
+}
+
+/* ==========  DARK THEME + AUTO ACCENT OVERRIDES ========== */
+
+/* Nav items with auto accent in dark mode: use off-white background with dark text */
+[data-theme='dark'][data-accent='auto'] .nav-item.active {
+  background: #e5e7eb;
+  color: #111827 !important;
+  box-shadow: var(--shadow-md);
+}
+
+[data-theme='dark'][data-accent='auto'] .nav-item.active .nav-icon,
+[data-theme='dark'][data-accent='auto'] .nav-item.active .nav-label {
+  color: #111827 !important;
 }