If you want to change default styling, this is complex because it's different for different browsers, and it does not work for all browsers. It can be done with custom CSS, but I can't make a tutorial about that here. See this tutorial:
https://css-tricks.com/the-current-state-of-styling-scrollbars-in-css/
You could try something like this in the custom.css
which would apply for most modern browsers:
.has-scrollbars::-webkit-scrollbar-thumb {
background-color: #AABBCC;
}
.has-scrollbars::-webkit-scrollbar-thumb:hover {
background-color: #112233;
}