update migration guide for 3.25

Author: saschadube

docs/pages/migration.md

@@ -24,6 +24,16 @@ Additionally, the accordion icon is now generated by a JavaScript component rath
 <a class="uk-accordion-title" href>Item 1 <span uk-accordion-icon></span></a>
 ```
 
+### Less Variables
+
+The following Less variables are also renamed. Just find and replace these Less variables if you are using a custom Less theme.
+
+| Less variable            | Replace with                          |
+| ------------------------ | ------------------------------------- |
+| `@form-padding-vertical` | `@form-multi-line-padding-vertical`   |
+
+***
+
 ## Migration from UIkit 3.23 to 3.24
 
 UIkit 3.24 refactors the [Countdown component](countdown.md). Almost all Less variables are removed. If you have customized the countdown using Less variables, you have to remove them. Simply search for `@countdown` in your Less files. Instead use text classes from the [Heading component](heading.md) or [Base component](base.md) to style the countdown.