Hi, Vladimir
The main menu attributes can be changed from Customizing->Navigations->Navigations primary menu.
https://prnt.sc/t6wwmz
Here are the codes to modify the submenu appearance:
/*Submenu background color*/
#masthead nav:not(.mobile-menu) ul.sub-menu {
background-color: black!important;
border-bottom-color: transparent!important;
}
/*lines between the submenu items*/
#masthead nav:not(.mobile-menu) ul.sub-menu li {
border-top-color: red!important;
border-bottom-color: red!important;
}
Change the colors as you wish.
Regards