Hi thibautgn,

There is not an option to change the shape of the submenu, but if you want then you can add a CSS code like this:

.main-header.menu-type-standard-menu .standard-menu-container.menu-skin-light div.menu>ul ul, .main-header.menu-type-standard-menu .standard-menu-container.menu-skin-light ul.menu ul {
    background: rgba(255, 255, 255, 0.9) !important;
    border-radius: 10px !important;
}

in your Custom CSS area.

Also to change the font-size of the sub-menus you can do that in the Typography area of the theme. It seems that your sub-menu fonts are already called in the Typography area because I can see that the selector:

.main-header.menu-type-standard-menu .standard-menu-container div.menu>ul ul li a, .main-header.menu-type-standard-menu .standard-menu-container ul.menu ul li a

is already calling font-size: 18px;

More info in this article Changing Font Sizes.

Have a great weekend!