To create a custom styled sidebar, go to the plugins tab and select Custom Scrollbar Styles. You must enter values for all of the following fields:
For all color fields, you can enter any HEX, RGB, RGBA, HSL, or HTML/CSS Color names. See below for more details.

-Width Pixels - Enter the number of pixels you want the scrollbar width to be. Ex: 10
-Thumb Color - Enter the color you want the scrollbar thumb to be (the portion of the scrollbar that slides up and down. Ex: blue, #b0bfe4, rgb(255, 182, 193), etc.
-Track Color - Enter the color you want for the entire track of the scrollbar to be (the track the thumb moves along).
-Thumb Border Radius  Pixels - The number of pixels to curve the thumb. Ex: 4
-Thumb Color On Hover - Set the color of the thumb when a user hovers over it. You can set this to the same color as the Thumb Color or a different color.
-Firefox Scrollbar Width - Set to auto, thin, or none. Auto is the normal Firefox scrollbar width.

You can use a color picker to find a color and its HEX, RGB, or HSL color code:

https://htmlcolorcodes.com/color-picker/

You can also enter any valid HTML/CSS color name:
https://gamingcolor.com/color-names-html-css