Scrollbars are default grey on light grey. Please make them inherit the color-scheme colors.
Described scrollbars are same as in chromium and can be changed with proper CSS. Or you can use native scrollbars (Preferences > Browsing > Use native scrollbars) but when these are enabled you can see some bugs...
Beware: native scroll bars dramatically effects the smoothness of scrolling (either with a touchpad or when dragging the scroll bar).
I tried putting the scrollbar CSS code from Breeze-GTK (https://github.com/KDE/breeze-gtk/blob/master/Breeze-gtk/gtk-3.20/gtk.css#L3181) into a User Style Sheet, and I found it looks pretty good. I've been meaning to play with it more to see if it could get even better. One caveat is that CSS scrollbars like this scale with the page zoom.
Correction to my previous comment: what works is the scrollbar CSS code from plasma-browser-integration (https://github.com/KDE/plasma-browser-integration/blob/master/extension/breeze-scroll-bars.css).