I'm not super knowledgeble on scrollbars, but i found the ones i use on
https://scripted.neocities.org/ !, i even figured out how to tweak the code of one to make the back of the bar transparent.
but from what i understand, it's very similar to border images! you need an image for your bumpers/up-down buttons, an image for the actual bar itself, one that can stretch as needed, and one for the background of the bar. Usually the images are pretty small, from what i've seen! You can also place the css inside a div for the bar to only apply there, or outside of all divs so it applies to the entire page!
w3 schools has a tutorial on the subject: https://www.w3schools.com/howto/howto_css_custom_scrollbar.asp and so do many other sites! i've lately preferred css tricks: https://css-tricks.com/almanac/properties/s/scrollbar/ for css related questionsi hope this helps!!