Hello. I have large 4K display. I had to scale icons in Preferences first. But what still annoys me is narrow scrollbars.
Had similar problem with other applications, e.g. Gwenview. Solved by modifying
.config/gtk-3.0/gtk.css.
But it has no effect on width of scrollbars in KiCad.
I tried running kicad binary with the use of strace tool to obtain list of gtk2 configuration files that it attempts to read.
strace -o trace ./kicad
found out that ~/.config/gtk-2.0/gtkrc and ~/.config/gtkrc are not opened at all,
but figured out that I need to put it in file named ~/.config/gtkrc-2.0. gtkrc-2.0 is file NOT a directory.
KiCad is now usable on my large 4K display, large icons, wide scrollbar. Nice.