Adds missing checkbox variables

src/themes/default/modules/checkbox.variables

@@ -162,6 +162,7 @@
 
 @toggleLaneBackground: @transparentBlack;
 @toggleLaneHeight: @toggleHandleSize;
+@toggleLaneBoxShadow: none;
 @toggleLaneVerticalOffset: 0rem;
 @toggleOffOffset: -0.05rem;
 @toggleOnOffset: (@toggleLaneWidth - @toggleHandleSize) + 0.15rem;
@@ -175,10 +176,13 @@
 @toggleHoverColor: @toggleFocusColor;
 
 @toggleOffLabelColor: @checkboxColor;
+@toggleOffHandleBoxShadow: none;
 
 @toggleOnLabelColor: @selectedTextColor;
 @toggleOnLaneColor: @primaryColor;
 
+@toggleOnHandleBoxShadow: none;
+
 @toggleOnFocusLaneColor: @primaryColorFocus;
 @toggleOnFocusLabelColor: @toggleOnLabelColor;