Hi there,

This is possible with some custom jQuery code. Please add the following via Appearance > Customize > Theme Options > Custom > Custom JS Footer:

<script> jQuery('.main-navigation li').addClass('nav-toggle-dropdown');</script>

If you don't want to use the submenu toggle function at all please use the following code instead. But this one should be added via Appearance > Customize > Additional CSS:

.main-navigation .nav-toggle-mobile-submenu { display: none; }
@media (max-width: 992px) { .main-navigation .sub-menu { display: block; } }