added checkbox image for white theme

src/css/less/forms.less

@@ -16,11 +16,11 @@ input[type="checkbox"]+.cr1 {
   margin-top: 4px;
   padding: 0 0 0 20px;
   vertical-align:middle;
-  background:url(../img/checkbox.png) left top no-repeat;
+  background: url(@checkboxImageUrl) left top no-repeat;
   cursor:pointer;
 }
 
 input[type="checkbox"]:checked+label {
-  background:url(../img/checkbox.png) 0px -18px no-repeat;
+  background: url(@checkboxImageUrl) 0px -18px no-repeat;
 }
 

src/css/less/variables.dark.less

@@ -290,7 +290,8 @@
 @popoverArrowOuterWidth:  @popoverArrowWidth + 1;
 @popoverArrowOuterColor:  rgba(0,0,0,.25);
 
-
+// images
+@checkboxImageUrl: '../img/checkbox.png';
 
 // GRID
 // --------------------------------------------------

src/css/less/variables.light.less

@@ -299,12 +299,12 @@
 @popoverArrowOuterWidth:  @popoverArrowWidth + 1;
 @popoverArrowOuterColor:  rgba(0,0,0,.25);
 
-
+// images
+@checkboxImageUrl: '../img/checkbox_white.png';
 
 // GRID
 // --------------------------------------------------
 
-
 // Default 940px grid
 // -------------------------
 @gridColumns:             12;