Trying to figure out how to make the border collapse on the primary navigation. I’m having to use CSS because there is no option to add a border in the customization that I see.
border-collapse only works when working with <table> elements, not lists unfortunately. As your navigation wraps to two lines i think the only solution would be to add this to your border CSS:
outline: 1px solid darkgrey;
With bother border and outline properties the line will be consistent but slightly thicker.
With just the outline property it will be thinner but you may notice the verticals are slightly thicker.