Why do I have two scroll bars in WordPress?
You're basically getting a double scrollbar because your giving the body min-height of 100vh AND setting an overflow. It appears this was done to keep the menu in the correct position on mobile devices.
How do I customize the scroll bar in WordPress?
Upon activation, you need to head over to Settings ยป Custom Color Scrollbar Settings page to configure plugin. From here, you can change the scrollbar color and the scrollbar rail background colors. You can then select the mouse scroll step, which is the scrolling speed of the mouse wheel.