.dropdown-submenu {
    position: relative;
}

    .dropdown-submenu a::after {
        transform: rotate(-90deg);
        position: absolute;
        right: 6px;
        top: .8em;
    }

    .dropdown-submenu .dropdown-menu {
        top: 0;
        left: 100%;
        margin-left: .1rem;
        margin-right: .1rem;
    }