fix(settings): restore theme toggle on OKX settings page

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/instance/templates/instance_theme_toggle.html

@@ -0,0 +1,12 @@
+<div class="theme-toggle instance-theme-toggle" role="group" aria-label="界面主题">
+    <button type="button" class="theme-toggle-btn is-active" data-theme-value="dark" aria-pressed="true" title="暗色主题">
+        <svg class="theme-icon" viewBox="0 0 24 24" width="18" height="18" aria-hidden="true">
+            <path fill="currentColor" d="M12.1 3a9 9 0 1 0 8.9 11 6.5 6.5 0 1 1-8.9-11z"/>
+        </svg>
+    </button>
+    <button type="button" class="theme-toggle-btn" data-theme-value="light" aria-pressed="false" title="亮色主题">
+        <svg class="theme-icon" viewBox="0 0 24 24" width="18" height="18" aria-hidden="true" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round">
+            <circle cx="12" cy="12" r="4"/><path d="M12 2v2M12 20v2M4.93 4.93l1.41 1.41M17.66 17.66l1.41 1.41M2 12h2M20 12h2M4.93 19.07l1.41-1.41M17.66 6.34l1.41-1.41"/>
+        </svg>
+    </button>
+</div>