numix-gtk-theme/gtk-3.0/scss/_window.scss

.window-frame {
	border: none;
	border-radius: $roundness $roundness 0 0;
	box-shadow: 0 10px 20px rgba(0,0,0,0.19), 0 6px 6px rgba(0,0,0,0.23),
				0 0 0 1px mix(shade($titlebar_bg_color, 0.7), $titlebar_fg_color, 0.21);

	/* this is used for the resize cursor area */
	margin: $spacing * 3;

	&:backdrop {
		box-shadow: 0 1px 3px rgba(0,0,0,0.12), 0 1px 2px rgba(0,0,0,0.24),
					0 0 0 1px mix(shade($titlebar_bg_color, 0.7), $titlebar_fg_color, 0.12);
	}

	&.tiled { border-radius: 0; }

	&.csd {
		&.popup {
			border-radius: 0;
			box-shadow: 0 3px 6px rgba(0,0,0,0.16), 0 3px 6px rgba(0,0,0,0.23),
						0 0 0 1px mix(shade($titlebar_bg_color, 0.7), $titlebar_fg_color, 0.21);
		}

		&.tooltip {
			border-radius: $roundness;
			box-shadow: none;
		}

		&.message-dialog {
			border-radius: $roundness;
			box-shadow: 0 3px 6px rgba(0,0,0,0.16), 0 3px 6px rgba(0,0,0,0.23),
						0 0 0 1px mix(shade($titlebar_bg_color, 0.7), $titlebar_fg_color, 0.21);
		}
	}
}