Add support (beta) for gtk 3.20.

gtk-3.20/gtk-widgets-assets-dark.css

@@ -0,0 +1,67 @@
+/*******************
+* check and radio *
+*******************/
+/* draw regular check and radio items using our assets */
+.check {
+	-gtk-icon-source: url("assets/checkbox-unchecked-dark.svg");
+}
+
+.check:insensitive {
+	-gtk-icon-source: url("assets/checkbox-unchecked-insensitive-dark.svg");
+}
+
+.check:checked {
+	-gtk-icon-source: url("assets/checkbox-checked-dark.svg");
+}
+
+.check:checked:insensitive {
+	-gtk-icon-source: url("assets/checkbox-checked-insensitive-dark.svg");
+}
+
+.check:inconsistent {
+	-gtk-icon-source: url("assets/checkbox-mixed-dark.svg");
+}
+
+.check:inconsistent:insensitive {
+	-gtk-icon-source: url("assets/checkbox-mixed-insensitive-dark.svg");
+}
+
+.radio {
+	-gtk-icon-source: url("assets/radio-unselected-dark.svg");
+}
+
+.radio:insensitive {
+	-gtk-icon-source: url("assets/radio-unselected-insensitive-dark.svg");
+}
+
+.radio:checked {
+	-gtk-icon-source: url("assets/radio-selected-dark.svg");
+}
+
+.radio:checked:insensitive {
+	-gtk-icon-source: url("assets/radio-selected-insensitive-dark.svg");
+}
+
+.radio:inconsistent {
+	-gtk-icon-source: url("assets/radio-mixed-dark.svg");
+}
+
+.radio:inconsistent:insensitive {
+	-gtk-icon-source: url("assets/radio-mixed-insensitive-dark.svg");
+}
+
+.menuitem.check {
+	-gtk-icon-source: none;
+}
+
+.menuitem.radio {
+	-gtk-icon-source: none;
+}
+
+GtkIconView.content-view.cell.check {
+	-gtk-icon-source: url("assets/grid-selection-unchecked-dark.svg");
+}
+
+GtkIconView.content-view.cell.check:active {
+	-gtk-icon-source: url("assets/grid-selection-checked-dark.svg");
+}