Add a variable to control input addon's background (#6040)

components/input/style/mixin.less

@@ -137,7 +137,7 @@
     line-height: 1;
     color: @input-color;
     text-align: center;
-    background-color: #eee;
+    background-color: @input-addon-bg;
     border: @border-width-base @border-style-base @input-border-color;
     border-radius: @border-radius-base;
     position: relative;

components/style/themes/default.less

@@ -236,6 +236,7 @@
 @input-color                 : @text-color;
 @input-border-color          : @border-color-base;
 @input-bg                    : #fff;
+@input-addon-bg              : #eee;
 @input-hover-border-color    : @primary-color;
 @input-disabled-bg           : @disabled-bg;