I’m not here to argue if this is or isn’t a customization. The way it adds the border is necessary for some of the styling for the sidebars and links. This is how the theme is designed and works, to change that for your specific needs is a custom use…
I’ve never adjusted this the way you’re asking, so I don’t know the answer to your question. I’ll go look it up myself and post some guidelines for the basic customization. I don’t completely understand the edit your making so if this should at least get you on the right track but you’ll need to adjust it to your specific need.