Hi,

Try to add the following code snippet to the Custom CSS area in Theme Settings.

body .mobile-nav .icon-sub-menu:before {
	content: "\e604";
	font-family: "simple-line-icons";
	color: white;
	font-size: 16px;
	margin-top: -7px;
	transform: none;
	-webkit-transform: none;
	background-color: transparent;
	width: auto;
	height: auto;
	left: 15px;
	right: auto;
}

body .mobile-nav .icon-sub-menu.up-icon:before {
	-webkit-transform: rotate(180deg);
	background-color: rotate(180deg);
	margin-top: -10px;
}

body .mobile-nav .icon-sub-menu:after {
	display: none;
}

Regards