Include window controls for CSD in theme and improve style

The icons are not loaded anymore from the icon-theme but are instead
included in the assets folder, so that the theme always looks consistent.
Furthermore, the backdrop window style has been tweaked.

gtk-3.0/gtk-widgets-assets.css

@@ -157,3 +157,90 @@ GtkIconView.content-view.cell.check:active, GtkIconView.content-view.cell.check:
     background-repeat: no-repeat;
     background-position: center center;
 }
+
+/*******************
+ * window controls *
+ *******************/
+.titlebar .button.titlebutton.close,
+.titlebar .button.titlebutton.close:backdrop:hover,
+.titlebar .button.titlebutton.close:backdrop:hover:focus {
+    background-image: url("assets/titlebutton-close.png");
+    background-repeat: no-repeat;
+    background-position: center center;
+    border: none;
+    box-shadow: none;
+    color: transparent;
+    icon-shadow: none;
+    opacity: 1;
+}
+
+.titlebar .button.titlebutton.close:hover,
+.titlebar .button.titlebutton.close:hover:focus {
+    background-image: url("assets/titlebutton-close-hover.png");
+}
+
+.titlebar .button.titlebutton.close:active,
+.titlebar .button.titlebutton.close:checked {
+    background-image: url("assets/titlebutton-close-active.png");
+}
+
+.titlebar .button.titlebutton.close:backdrop {
+    background-image: url("assets/titlebutton-close.png");
+    opacity: 0.4;
+}
+
+.titlebar .button.titlebutton.maximize,
+.titlebar .button.titlebutton.maximize:backdrop:hover,
+.titlebar .button.titlebutton.maximize:backdrop:hover:focus {
+    background-image: url("assets/titlebutton-maximize.png");
+    background-repeat: no-repeat;
+    background-position: center center;
+    border: none;
+    box-shadow: none;
+    color: transparent;
+    icon-shadow: none;
+    opacity: 1;
+}
+
+.titlebar .button.titlebutton.maximize:hover,
+.titlebar .button.titlebutton.maximize:hover:focus {
+    background-image: url("assets/titlebutton-maximize-hover.png");
+}
+
+.titlebar .button.titlebutton.maximize:active,
+.titlebar .button.titlebutton.maximize:checked {
+    background-image: url("assets/titlebutton-maximize-active.png");
+}
+
+.titlebar .button.titlebutton.maximize:backdrop {
+    background-image: url("assets/titlebutton-maximize.png");
+    opacity: 0.4;
+}
+
+.titlebar .button.titlebutton.minimize,
+.titlebar .button.titlebutton.minimize:backdrop:hover,
+.titlebar .button.titlebutton.minimize:backdrop:hover:focus {
+    background-image: url("assets/titlebutton-minimize.png");
+    background-repeat: no-repeat;
+    background-position: center center;
+    border: none;
+    box-shadow: none;
+    color: transparent;
+    icon-shadow: none;
+    opacity: 1;
+}
+
+.titlebar .button.titlebutton.minimize:hover,
+.titlebar .button.titlebutton.minimize:hover:focus {
+    background-image: url("assets/titlebutton-minimize-hover.png");
+}
+
+.titlebar .button.titlebutton.minimize:active,
+.titlebar .button.titlebutton.minimize:checked {
+    background-image: url("assets/titlebutton-minimize-active.png");
+}
+
+.titlebar .button.titlebutton.minimize:backdrop {
+    background-image: url("assets/titlebutton-minimize.png");
+    opacity: 0.4;
+}

gtk-3.0/gtk-widgets.css

@@ -2081,6 +2081,16 @@ OsScrollbar:insensitive {
     color: shade(@titlebar_bg_color, 0.8);
 }
 
+.header-bar GtkSeparatorToolItem:backdrop,
+.header-bar .separator:backdrop,
+.header-bar .separator:backdrop:insensitive,
+.titlebar GtkSeparatorToolItem:backdrop,
+.titlebar .separator:backdrop,
+.titlebar .separator:backdrop:insensitive {
+    border-color: shade(@titlebar_bg_color, 0.95);
+    color: shade(@titlebar_bg_color, 0.95);
+}
+
 .primary-toolbar .button .separator,
 .primary-toolbar .button.separator,
 .toolbar .button .separator,
@@ -4374,8 +4384,8 @@ GtkBubbleWindow .toolbar {
 
 .titlebar:backdrop {
     background-image: linear-gradient(to bottom,
-                                      shade(@titlebar_bg_color, 1.02),
-                                      shade(@titlebar_bg_color, 0.88)
+                                      shade(@titlebar_bg_color, 1.12),
+                                      shade(@titlebar_bg_color, 0.98)
                                       );
 }
 
@@ -4384,6 +4394,10 @@ GtkBubbleWindow .toolbar {
     text-shadow: 0 1px shade(@titlebar_bg_color, 1.05);
 }
 
+.titlebar .button:backdrop {
+    color: mix(@titlebar_fg_color, @titlebar_bg_color, 0.6);
+}
+
 .titlebar .titlebutton {
     padding: 4px;
     border-style: none;