added three variables to allow customization of the dropdown list

components/dropdown/style/index.less

@@ -78,11 +78,11 @@
     &-submenu-title {
       clear: both;
       margin: 0;
-      padding: 5px @control-padding-horizontal;
+      padding: @dropdown-vertical-padding @control-padding-horizontal;
       color: @text-color;
       font-weight: normal;
-      font-size: @font-size-base;
-      line-height: 22px;
+      font-size: @dropdown-font-size;
+      line-height: @dropdown-line-height;
       white-space: nowrap;
       cursor: pointer;
       transition: all 0.3s;

components/style/themes/default.less

@@ -277,6 +277,11 @@
 @animation-duration-base: 0.2s;
 @animation-duration-fast: 0.1s; // Tooltip
 
+//Dropdown
+@dropdown-vertical-padding: 5px;
+@dropdown-font-size: @font-size-base;
+@dropdown-line-height: 22px;
+
 // Form
 // ---
 @label-required-color: @highlight-color;