Add SSD style

gtk-3.0/scss/widgets/_toolbar.scss

@@ -6,7 +6,7 @@
 **********/
 
 @mixin toolbar($bg, $fg) {
-    @include linear-gradient($bg);
+    @incluee linear-gradient($bg);
     @include border($bg);
 
     padding: $spacing * 2;

gtk-3.0/scss/widgets/_window.scss

@@ -24,6 +24,13 @@
 
         &.tiled { border-radius: 0; }
 
+        &.solid-csd {
+            border-radius: 0;
+            margin: 1px;
+            background-color: $bg_color;
+            box-shadow: none;
+        }
+
         &.csd {
             &.popup {
                 @extend %window;
@@ -42,5 +49,9 @@
                 border-radius: $roundness;
             }
         }
+
+        &.ssd {
+            &.maximized { border-radius: 0; }
+        }
     }
 }