theme: Reduce the height of titlebars

The height of our titlebars is geared towards headerbars that can
host controls. For a traditional title-only titlebar, a smaller
footprint is sufficient and frees up some valuable vertical space

gtk/theme/Default/_common.scss

@@ -1555,21 +1555,21 @@ headerbar {
   }
 
   &.default-decoration {
-    min-height: 28px;
+    min-height: 0px;
-    padding: 4px;
+    padding: 0px 2px;
 
     windowcontrols {
       button,
       menubutton {
-        min-height: 26px;
+        min-height: 18px;
-        min-width: 26px;
+        min-width: 18px;
         margin: 0;
         padding: 0;
       }
 
       menubutton button {
-        min-height: 20px;
+        min-height: 18px;
-        min-width: 20px;
+        min-width: 18px;
         margin: 0;
         padding: 4px;
       }