As a result, the desktop version did not look as desired. Have now made the setting you mentioned and adjusted the rest with SCSS.
Desktop

Mobile

Code (SCSS)
$mobile-menu-breakpoint: 960px;
.main-navigation{
border-bottom: 1px solid var(--border);
@media (min-width: $mobile-menu-breakpoint){
border-bottom: none;
}
#primary-menu{
border-top: 1px solid var(--border);
border-bottom: 1px solid var(--border);
@media (min-width: 1px) and (max-width: $mobile-menu-breakpoint){
border-bottom: none;
}
}
#primary-menu li a{
text-align: center;
}
@media (min-width: $mobile-menu-breakpoint){
#primary-menu li:hover a{
font-weight: 700;
-webkit-box-shadow: inset 0px -2px 0px 0px rgba(0,0,0,1);
-moz-box-shadow: inset 0px -2px 0px 0px rgba(0,0,0,1);
box-shadow: inset 0px -2px 0px 0px rgba(0,0,0,1);
}
}
}
@media (min-width: $mobile-menu-breakpoint) {
.navigation-branding, .main-nav{
width: 100%;
justify-content: center;
}
#menu-hauptmenu{
justify-content: center;
}
}