fix: 更新余额显示样式以支持深色模式

frontend/src/components/admin/user/UserBalanceModal.vue

@@ -13,7 +13,7 @@
         </div>
       </div>
       <div><label class="input-label">{{ t('admin.users.notes') }}</label><textarea v-model="form.notes" rows="3" class="input"></textarea></div>
-      <div v-if="form.amount > 0" class="rounded-xl border border-blue-200 bg-blue-50 p-4"><div class="flex items-center justify-between text-sm"><span>{{ t('admin.users.newBalance') }}:</span><span class="font-bold">${{ formatBalance(calculateNewBalance()) }}</span></div></div>
+      <div v-if="form.amount > 0" class="rounded-xl border border-blue-200 bg-blue-50 p-4 dark:border-blue-800 dark:bg-blue-950"><div class="flex items-center justify-between text-sm"><span class="text-gray-700 dark:text-gray-300">{{ t('admin.users.newBalance') }}:</span><span class="font-bold text-gray-900 dark:text-gray-100">${{ formatBalance(calculateNewBalance()) }}</span></div></div>
     </form>
     <template #footer>
       <div class="flex justify-end gap-3">