We can do that but then active menu color needs to be indicated in some way , also create course button will not be distinctive .

but still you want it then : 

Please try adding this custom css in your wp-admin-> appearance-> customize -> custom css : 

a.menu-child {
    -webkit-appearance: none;
    align-items: center;
    border: 1px solid transparent;
    border-radius: 4px;
    box-shadow: none;
    font-size: 1rem;
    height: 2.25em;
    justify-content: flex-start;
    line-height: 1.5;
    padding-bottom: calc(0.375em - 1px);
    padding-left: calc(0.625em - 1px);
    padding-right: calc(0.625em - 1px);
    padding-top: calc(0.375em - 1px);
    position: relative;
    vertical-align: top;
    margin:0.5rem 0 !important;
    background: var(--highlight) !important;
    color: var(--primarycolor) !important;
    text-align:center !important;
    display:block !important;
    twrd-wrap: initial!important;
    height:auto;
}

a.menu-child.current-menu-item{
    background: var(--primary) !important;
}