theme: small round button for system menu actions

- got freeze break from release team (2) and the docs team

https://bugzilla.gnome.org/show_bug.cgi?id=706638

data/theme/gnome-shell.css

@@ -655,18 +655,27 @@ StScrollBar StButton#vhandle:active {
 
 .system-menu-action {
     color: #e6e6e6;
-    border-radius: 4px;
-    padding: 6px;
+    border-radius: 32px; /* wish we could do 50% */
+    padding: 13px;
+    border: 1px solid #5f5f5f; /* using rgba() is flaky unfortunately */
 }
 
 .system-menu-action:hover,
 .system-menu-action:focus {
     color: white;
-    background-color: rgba(255,255,255,0.1);
+    background-color: #4c4c4c;
+    border: none;
+    padding: 14px;
+}
+
+.system-menu-action:active {
+    color: black;
+    background-color: #6f6f6f;
+
 }
 
 .system-menu-action > StIcon {
-    icon-size: 32px;
+    icon-size: 16px;
 }
 
 .screencast-indicator {