To customize Oxygen Builder Off-canvas responsive dropdown Menu, insert below CSS in your custom stylesheet.

/* Medium Devices, Desktops */
@media only screen and (max-width : 992px){

.menu-item a {
  font-size: 22px!important;
}
.menu-item-has-children .menu-item a {
  font-size: 18px!important;
  padding-top: 2px!important;
  padding-bottom: 2px!important;
}
.menu-item-has-children .menu-item a::before {
  content:'-';
  padding-right: 6px;
}

}