Hi @ismaine
Thank you for reaching out to us. You can do this by assigning a class to your menu item by navigating to Appearance > Menus then expand your menu item and in the CSS Classes (optional) field enter standout-menu-item (see screenshot)

If you don’t see the CSS Classes (optional) field you’ll need to first enable it by clicking on Screen Options in the upper right corner of the screen (see screenshot)

In Show advanced menu properties section enable CSS Classes option (see screenshot)

You can then add the following code in the Theme Options > CSS:
.standout-menu-item > a > span {
border: 2px solid #559D07;
padding: 20px;
background: #c2ffb5;
color: #559D07;
}
Here are some related links for further reading, this could help you in finding and implementing some CSS fixes:
Don’t forget to clear all caches including your browser’s cache after adding the code. Let us know how this goes!