Border around one menu item

please post in http://ww.wp.xz.cn/support/theme/quark/

try using a browser inspection tool to identify which CSS selectors you can use to format the menu item.

Try this as a starting point:

#menu-item-736 {
  border:1px solid #333333;
}

Evan

where exactly have you added the new style?

consider using a ‘custom CSS’ plugin for edits on the formatting; http://ww.wp.xz.cn/plugins/search.php?q=custom+css

your style.css is broken near the very end, through this unclosed style:

.firefox img, .firefox a img{
width: 98%;