[Daunk]

That is not XFCE itself, that is up to your theme. I'm on the Dracula theme and have about 16 pixels.

[hxorr]

Are there any themes that allow changing window border size, title bar size, scrollbar width etc, like win 9x?

[ReallyOldLurker]

My scrollbar changes in Debian:

1. as root, edit /etc/environment and add: GTK_OVERLAY_SCROLLING=0

2. in ~/.config/gtk-3.0/gtk.css add: .scrollbar.vertical slider, scrollbar.vertical slider {min-width: 10px;}