firefox-gnome-theme/theme/parts/sidebar.css

/* Sidebar */

@namespace xul "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";

#sidebar-box {
	background: var(--gnome-sidebar-background) !important;
}

#sidebar-header {
	background: var(--gnome-toolbar-background) !important;
	border-bottom: 1px solid var(--gnome-toolbar-border-color) !important;
	padding: 6px !important;
}
#sidebar-search-container {
	padding: 6px !important;
}

#sidebar-box:-moz-window-inactive label, #sidebar-box:-moz-window-inactive image,
.sidebar-panel:-moz-window-inactive label, .sidebar-panel:-moz-window-inactive image,
.sidebar-placesTreechildren:-moz-window-inactive {
	opacity: 0.7 !important;
}
#sidebar-splitter {
	background: var(--gnome-toolbar-background) !important;
	border: 0 !important;
	border-right: 1px solid var(--gnome-border-color) !important;
	width: 0 !important;
}