Start with reading the uicustomization docs. Also the CSS specification will help.
Scrollbar style can be read here from the default theme.

You can override those rules in your style.css, for example the size:
The scrollbar is a quite complex element because it contains several child elements, but,
hback abd vback the background of the slider (horizontal and vertical),
hbutton and vbutton are the sliders.

ScrollBar {
	min-width: 24dp;
	min-height: 24dp;
}

ScrollBar::hback {
	min-height: 20dp;
}

ScrollBar::vback {
	min-width: 20dp;
}

ScrollBar::hbutton {
	min-height: 20dp;
}

ScrollBar::vbutton {
	min-width: 20dp;
}