Wayfinder by default already gives the last item a class of "last".
.last {margin-left: 20px;}
will do the trick. Be more specific in the css rule to control inner rows:
ul li ul li.last {margin-left: 0;}
will prevent inner items from inheriting the margin. Never forget that these are Cascading Style Sheets; the rules you set "cascade" down the HTML structure!
To what form are you referring? Wayfinder has nothing to do with forms.
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