
This code will give your off-canvas menu a solid border on the left side of the item that is “current page”.
You may need to adjust the color and positioning.
@media (max-width: 1140px){
.main-navigation .main-nav ul li[class*="current-menu-"] > a:before{
content: '';
position: absolute;
margin-left: -24px;
background-color: var(--accent);
width: 4px;
height: 100%;
}
}