addressing #9189 manually, part 4: input group background var

less/input-groups.less

@@ -49,7 +49,7 @@
   font-weight: normal;
   line-height: 1;
   text-align: center;
-  background-color: @gray-lighter;
+  background-color: @input-group-addon-bg;
   border: 1px solid @input-group-addon-border-color;
   border-radius: @border-radius-base;
 

less/variables.less

@@ -147,6 +147,7 @@
 @legend-color:                   @gray-dark;
 @legend-border-color:            #e5e5e5;
 
+@input-group-addon-bg:           @gray-lighter;
 @input-group-addon-border-color: @input-border;