To remove the border use this code in your CSS:
@media screen and (min-width: 59.6875em) {
body:before {
box-shadow: none;
}
}
Explanation:
The border to the sidebar is added with :before pseudo-class so you have to remove it on the body and only for the higher resolution (because of that you will use media query).
If you don’t want to use this in your theme style.css you can use “Modular Custom CSS” plugin that will let you add custom css code to your Twenty Fifteen theme.
Hope this helps 🙂
