OVERFLOW-Y IS NOT AUTOMATIC IN A FIXED DIV
overflow-y is not visible by default if your div is fixed in position and size. You may want to set it manually to auto
CHANGING THE SIDEBAR YOU NEED TO SET EVERYTHING UP
You can not change the width of your sidebar, if you do that, the sidebar will disappear.
You need to apply others styles too: https://www.w3schools.com/howto/howto_css_custom_scrollbar.asp