When theming menu components, no SASS variables exist to modify the menu background gradient style or border width and radius. Instead, the values are hard-coded to use a "matte" gradient, a border width of 1px, and a border radius of 3px. From resources/themes/stylesheets/ext4/default/widgets/_menu.scss:
Code:
    .#{$prefix}menu-item-active {
        cursor: pointer;

        .#{$prefix}menu-item-link {
            @include background-gradient($menu-item-active-background-color, 'matte');
            margin: 0px;
            border: 1px solid $menu-item-active-border-color;
            cursor: pointer;
            @include border-radius(3px);
        }
    }
Variables such as $menu-item-active-background-gradient and $menu-item-active-border-radius should be added to allow these values to be changed for individual themes.