Prevent GTK+3 applications (Firefox, PCManFM) from showing razor-thin scrollbars.

Save the following as ~/.config/gtk-3.0/gtk.css:

/* https://askubuntu.com/questions/775201/how-do-i-get-a-bigger-static-scrollbar-aka-normal-scrollbar */
scrollbar slider {
    /* Size of the slider */
    min-width: 20px;
    min-height: 20px;
    border-radius: 22px;
 
    /* Padding around the slider */
    border: 5px solid transparent;
}

Source: Ask Ubuntu - How do I get a bigger static scrollbar (aka normal scrollbar)?