How do I enable the gradient buttons in BS4 as BS3 has using its theme.css?

Dear community
I want to be able to give to buttons the same gradients as BS3 has, using its bootstrap-theme.min.css
<img src="http://i.imgur.com/jNwZ6ce.jpg" alt="">
I've enabled _$enable-gradients_ to **true** in my compilation but I don't see any effect on them.
<img src="http://i.imgur.com/74V36Zu.jpg" alt="">
What am I doing wrong?
Regards
