If you need to change the style, you need to change the CSS. Mendix uses SCSS to do this, and it is recommended to use Calypso to do this. There is a good article on how to set this up and make changes that I recommend you look at. https://docs.mendix.com/howto/front-end/calypso
For CSS styling of scrollbars, there is a good article on the CSS Tricks website. https://css-tricks.com/the-current-state-of-styling-scrollbars/
Hope this helps.
Thank you robert,
I have created one scss file in my project file and converted to css but still these styles are not getting apply to my page scrollbar. help me out