Reduce Drop down Menu Width in WordPress

It’s a css issue. You need to edit two selectors.

First is the menu width. It’s set to 100% which is why you’re seeing black space. Edit this class below.:

.cap-primary-menu .menu > li.menu-full-width .cap-submenu-ddown {
    padding: 20px 25px 25px;
    width: 100%; // set this to auto or something smaller than 100%
    left: 0;
    border: 1px solid #eee;
    background: #fff;
}

The li tags within that menu are set to be divisible by five, minus some padding, which is why you see 18%. So you need to change this selector:

.cap-primary-menu .menu > li.menu-full-width .cap-submenu-ddown .container > ul > li {
    width: 18.297%; // change this to 30% and go from there.
}