Depois de mais algumas pesquisas, descobri que esses indicadores são chamados de overshoot e undershoot.
In GTK 3.16 (for example from ppa:ubuntu-desktop/ww), GtkScrolledWindows indicate whether there is content that can be scrolled to (Adwaita shows a dashed line for this) and if the user scrolls when there is no more content (Adwaita shows a gradient thing). These are called undershoot and overshoot respectively.
Ambiance and Radiance don't style these classes, and the default is to show an opaque grey area. This looks poor.
Para corrigir o problema no fedora com light-gtk3-theme v14.04
i, adicionei este CSS a /usr/share/themes/Ambiance/gtk-3.0/gtk-widgets.css:
/*************
* overshoot *
*************/
.overshoot.top {
background: -gtk-gradient(radial, center top, 0, center top, 0.7, from(shade(@bg_color, 0.92)), to(alpha(@bg_color, 0.0)));
}
.overshoot.right {
background: -gtk-gradient(radial, right center, 0, right center, 0.7, from(shade(@bg_color, 0.92)), to(alpha(@bg_color, 0.0)));
}
.overshoot.bottom {
background: -gtk-gradient(radial, center bottom, 0, center bottom, 0.7, from(shade(@bg_color, 0.92)), to(alpha(@bg_color, 0.0)));
}
.overshoot.left {
background: -gtk-gradient(radial, left center, 0, left center, 0.7, from(shade(@bg_color, 0.92)), to(alpha(@bg_color, 0.0)));
}
/**************
* undershoot *
**************/
.undershoot {
background: none;
}
Veja revisão 436 para o Radiance CSS.