In Kale, you can choose between displaying a default scrollbar or a Fancy Scrollbar - specifically designed to fit perfectly with the design of the theme. To choose your preference, go to Appearance > Customize > Enable Fancy Scrollbar? and toggle the appropriate choice.
If you choose to enable the Fancy Scrollbar, it will appear as a stylish dark-grey bar.
If you disable it, the scrollbar will look the same as your browser's default one.