Add dark theme vars

1、@ve-dropdown-operation-item-text-color
2、@ve-dropdown-operation-item-hover-text-color

Author: huangshuwei

packages/style/ve-dropdown.less

@@ -182,14 +182,14 @@
         justify-content: space-around;
 
         .ve-dropdown-operation-item {
-            color: #495060;
+            color: @ve-dropdown-operation-item-text-color;
 
             &.ve-dropdown-filter-disable {
                 color: #999;
             }
 
             &:not(.ve-dropdown-filter-disable):hover {
-                color: @ve-primary-color;
+                color: @ve-dropdown-operation-item-hover-text-color;
             }
         }
 

packages/theme-dark/var.less

@@ -66,6 +66,8 @@
 @ve-dropdown-item-hover-background-color: #464646;
 @ve-dropdown-item-hover-text-color: @ve-dark-text-color;
 @ve-dropdown-active-text-color: @ve-dark-text-color;
+@ve-dropdown-operation-item-text-color: @ve-dark-text-color;
+@ve-dropdown-operation-item-hover-text-color: #2196f3;
 
 /* ve-contextmenu */
 @ve-contextmenu-background-color: @ve-secondary-color;

packages/theme-default/var.less

@@ -65,6 +65,8 @@
 @ve-dropdown-item-hover-background-color: #f3f3f3;
 @ve-dropdown-item-hover-text-color: @ve-primary-color;
 @ve-dropdown-active-text-color: @ve-primary-color;
+@ve-dropdown-operation-item-text-color: #495060;
+@ve-dropdown-operation-item-hover-text-color: @ve-primary-color;
 
 /* ve-contextmenu */
 @ve-contextmenu-background-color: #fff;