nathanshel Try add this to custom.css file and change CSS properties accordingly :
a.bg-primary{padding:10px !important;color:#000 !important;}
Kindly ensure to clear your browser cache if changes dont reflect.
Element styling for specific menu item/page
If wishing to get the styling as mentioned in title, You may enter page class suffix assuming btn-color for specific menu item following tutorial here and thereon, add this in custom.css file :
.btn-color a.bg-primary{padding:10px !important;color:#000 !important;}
Note: Do ensure to add space before suffix being added while adding it in specific text field
