[HTML/CSS] Dropdown에 경계선 추가하기

이 경계선을 추가하고 싶다면?

- 전체 Navbar 코드 from bootstrap

<nav class="navbar navbar-expand-lg navbar-dark bg-dark">

<a class="navbar-brand" href="#">Navbar</a>

<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">

<span class="navbar-toggler-icon"></span>

</button>

<div class="collapse navbar-collapse" id="navbarSupportedContent">

<ul class="navbar-nav mr-auto">

<li class="nav-item active">

<a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>

</li>

<li class="nav-item">

<a class="nav-link" href="{% url 'new' %}">글쓰기</a>

</li>

<li class="nav-item dropdown">

<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">

Dropdown

</a>

<div class="dropdown-menu" aria-labelledby="navbarDropdown">

<a class="dropdown-item" href="{% url 'portfolio' %}">portfolio</a>

<a class="dropdown-item" href="#">Another action</a>

<div class="dropdown-divider"></div>

<a class="dropdown-item" href="#">Something else here</a>

</div>

</li>

<li class="nav-item">

<a class="nav-link disabled" href="#" tabindex="-1" aria-disabled="true">Disabled</a>

</li>

</ul>

<form class="form-inline my-2 my-lg-0">

<input class="form-control mr-sm-2" type="search" placeholder="Search" aria-label="Search">

<button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>

</form>

</div>

</nav>