Fix horizontal arrow positioning

[alexmansfield]

Change arrow placement to use left: 50%; margin-left: -5px; so that the arrow will be centered regardless of the width of the :after element. Like this:

    .toggle-submenu:after {
        content: '';
        position: absolute;
        top: 50%;
        left: 50%;
        margin-top: -3px;
        margin-left: -5px;
        height: 0;
        width: 0;
    }