since at least at my device a "c:\ dir /S userchrome.css" had no result...
Introduction to customising Joplin (userchrome.css & userstyle.css) could be a good introduction.
and
How To Style Scrollbars with CSS | DigitalOcean
=> go to Tools => Options
- Appearance
- "show Advanced settings"
- "Custom stylesheet for Joplin-wide settings" <= this is userchrome.css
