active link

a:active  {     background-color: yellow;  }

# Foward slash is equal to homepage. For all others you use IN keyword.
<li 
  {% if '/' == request.path %}
  	class="nav-item active mr-3"
  {% else %}
  	class="nav-item mr-3"
  {% endif %}
>

<li 
  {% if 'about' in request.path %}
      class="nav-item active mr-3"
  {% else %}
      class="nav-item mr-3"
  {% endif %}
>