https://askubuntu.com/questions/775201/how-to-get-a-bigger-static-scroll...
Edited file /usr/share/themes/Ambiance/gtk-3.0/gtk-widgets.css
I changed -GtkRange-slider-width: 25; (original value was 10)
Also changed margin-left, margin-right and margin-top as shown. Change all three of these from 2px to 7px
Problem solved. !!!
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