This changes various parts of the Divi submenu

/*** -- START submenu changes-- ***/
/* -- add submenu outer borders */
.sub-menu {
border-color: #504C4C !important;
border-style: solid !important;
border-width: 1px !important;
}
/* -- add submenu item bottom border */
.sub-menu li {
border-bottom-color: #504C4C;
border-bottom-style: solid;
border-bottom-width: 2px;
}
/* -- remove submenu outer spacing */
#top-menu li li {
    margin: 0;
    padding: 0;
}
/* -- remove submenu item spacing */
.nav li ul {
padding: 0;
width: 200px;
}
/* -- change submenu item background hover color*/
.nav li li:hover {
background-color: rgba(80,76,76,0.1);
}
/*** -- END submenu changes-- ***/

Code language: CSS (css)