themes: Define popover button padding

The padding for image buttons on popovers like the layer lock
options was not defined. On system themes which set this padding
to 0px, it made it difficult to tell if the alpha lock was enabled
or not. This patch defines the padding to prevent this issue.

themes/Default/common.css

@@ -568,6 +568,11 @@ border {
   color: @fg-color;
 }
 
+/* Define popover image button padding */
+popover button {
+  padding: 4px 9px;     
+}
+
 /* Top menu items  */
 
 /* give the menu bar a color, fix for the default color being too dark */