Added assets for dark theme and tweaked assets. Fixes https://github.com/cldx/numix-gtk-theme/issues/4

gtk-3.0/gtk-dark.css

@@ -58,7 +58,7 @@
 @define-color wm_border_unfocused #393939;
 
 @import url("gtk-widgets.css");
-@import url("gtk-widgets-assets.css");
+@import url("gtk-widgets-assets-dark.css");
 @import url("apps/gnome-applications.css");
 @import url("apps/granite-widgets.css");
 @import url("apps/unity.css");

gtk-3.0/gtk-widgets-assets-dark.css

@@ -0,0 +1,114 @@
+/*******************
+ * check and radio *
+ *******************/
+/* draw regular check and radio items using our assets */
+.check,
+.check row:selected,
+.check row:selected:focus {
+    background-image: url("assets/checkbox-unchecked-dark.png");
+}
+
+.check:insensitive,
+.check row:selected:insensitive,
+.check row:selected:focus:insensitive {
+    background-image: url("assets/checkbox-unchecked-insensitive-dark.png");
+}
+
+.check:active,
+.check row:selected:active,
+.check row:selected:focus:active {
+    background-image: url("assets/checkbox-checked-dark.png");
+}
+
+.check:active:insensitive,
+.check row:selected:active:insensitive,
+.check row:selected:focus:active:insensitive {
+    background-image: url("assets/checkbox-checked-insensitive-dark.png");
+}
+
+.check:inconsistent,
+.check row:selected:inconsistent,
+.check row:selected:focus:inconsistent {
+    background-image: url("assets/checkbox-mixed-dark.png");
+}
+
+.check:inconsistent:insensitive,
+.check row:selected:inconsistent:insensitive,
+.check row:selected:focus:inconsistent:insensitive {
+    background-image: url("assets/checkbox-mixed-insensitive-dark.png");
+}
+
+.radio,
+.radio row:selected,
+.radio row:selected:focus {
+    background-image: url("assets/radio-unselected-dark.png");
+}
+
+.radio:insensitive,
+.radio row:selected:insensitive,
+.radio row:selected:focus:insensitive {
+    background-image: url("assets/radio-unselected-insensitive-dark.png");
+}
+
+.radio:active,
+.radio row:selected:active,
+.radio row:selected:focus:active {
+    background-image: url("assets/radio-selected-dark.png");
+}
+
+.radio:active:insensitive,
+.radio row:selected:active:insensitive,
+.radio row:selected:focus:active:insensitive {
+    background-image: url("assets/radio-selected-insensitive-dark.png");
+}
+
+.radio:inconsistent,
+.radio row:selected:inconsistent,
+.radio row:selected:focus:inconsistent {
+    background-image: url("assets/radio-mixed-dark.png");
+}
+
+.radio:inconsistent:insensitive,
+.radio row:selected:inconsistent:insensitive,
+.radio row:selected:focus:inconsistent:insensitive {
+    background-image: url("assets/radio-mixed-insensitive-dark.png");
+}
+
+.menuitem.check:active {
+    background-image: url("assets/menuitem-checkbox-checked.png");
+}
+
+.menuitem.check:active:hover {
+    background-image: url("assets/menuitem-checkbox-checked-hover.png");
+}
+
+.menuitem.check:active:insensitive {
+    background-image: url("assets/menuitem-checkbox-checked-insensitive.png");
+}
+
+.menuitem.check:inconsistent:hover,
+.menuitem.radio:inconsistent:hover {
+    background-image: url("assets/menuitem-checkbox-mixed-hover.png");
+}
+
+.menuitem.check:inconsistent,
+.menuitem.radio:inconsistent {
+    background-image: url("assets/menuitem-checkbox-mixed.png");
+}
+
+.menuitem.check:inconsistent:insensitive,
+.menuitem.radio:inconsistent:insensitive {
+    background-image: url("assets/menuitem-checkbox-mixed-insensitive.png");
+}
+
+.menuitem.radio:active {
+    background-image: url("assets/menuitem-radio-checked.png");
+}
+
+.menuitem.radio:active:hover {
+    background-image: url("assets/menuitem-radio-checked-hover.png");
+}
+
+.menuitem.radio:active:insensitive {
+    background-image: url("assets/menuitem-radio-checked-insensitive.png");
+}