I tried to change submenu’s border-top color, but didnt find where to do that on the theme settings, so i tried to do that with custom css. I inputted following to the custom css field in theme’s settings:

#menu > ul > li > div {
border-top: 2px solid #fff;
}
.custom_menu_4 #menu > ul > li ul > li > a:hover {
background-color: #fff;
}

css does not work even if i put !important

the place is – Appearance->Customize->Main Menu Style->Navigation Accent Color.
This is the active color for your menu and it matches the hover item color. If you want a different color than the hover state you can surely do it with CSS.

#menu > ul > li div {
    border-top: 2px solid yellow!important;
}

Where are you adding the CSS? The above should work.

I tried with your example with no result (Fortuna options->Custom CSS)

#menu > ul > li div {
    border-top: 2px solid yellow!important;
}

What’s you URL, please, where you added it?

Do you need the admin password?

It’s the “>” sign that got HTML encoded to “& gt;” you can see it in your code!
Try again removing the “>”, in the mean time I will contact the Option Tree plugin vendor for a fix:

#menu ul li div {
    border-top: 2px solid yellow!important;
}