the bordered sidebar

@ayoungerskin

Sure, try this in Appearance > Customize > Additional CSS

.sidebar-borders {
    border: thin solid #1a1a1a;
}

Thank you! And if I want it even a bit thinner? ??

Actually border set to thin is the smallest one. You can use value in pixels to have more control, e.g.

.sidebar-borders {
    border: 2px solid #1a1a1a;
}

Best