Fix one pixel offset on inverted menu due to lack of border

src/definitions/collections/menu.less

@@ -1241,7 +1241,7 @@ Floated Menu / Item
 ---------------*/
 
 .ui.inverted.menu {
-  border-width: 0px;
+  border: @invertedBorder;
   background: @invertedBackground;
   box-shadow: @invertedBoxShadow;
 }

src/themes/default/collections/menu.variables

@@ -370,6 +370,7 @@
 /* Inverted */
 @invertedBackground: @black;
 @invertedBoxShadow: none;
+@invertedBorder: 1px solid transparent;
 @invertedHeaderBackground: transparent;
 
 @invertedItemBackground: transparent;