btn-default class in bootstrap 4
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
<div class="container m-3">
<a class="btn btn-outline-secondary">outline-secondary link</a>
<button type="button" class="btn btn-outline-secondary">outline-secondary 'button'</button>
<br><br>
<a class="btn btn-outline-light">outline-light link</a>
<button type="button" class="btn btn-outline-light">outline-light 'button'</button>
</div>