Hi Serina,
You need to write the custom css to fix it, simply you can use css code below and you can modify it as your require icons and color.
ul.navmenu li.menu-item-has-children:before {
content: '\25bc';
float: left;
font-size: 9px;
line-height: 41px;
margin-left: 5px;
color: #7bbe3b;
}
Here is the link for your reference : http://www.smashingmagazine.com/2011/07/learning-to-use-the-before-and-after-pseudo-elements-in-css/
Note: use the custom css plugin or child theme stylesheet to write the custom css.
Thanks