all you need to do is, just add some color on active slide’s indicator, I have used your code, and aaded a single line in css, and you can see the result.
body {background: #333 !important;}
.carousel-indicators {
bottom: -50px !important;
}
.carousel-indicators li.active {background: red;}
.carousel-item {color: white; font-size: 80px;}<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.4.0/js/bootstrap.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/2.4.0/umd/popper.min.js"></script>
<link href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.4.1/css/bootstrap.min.css" rel="stylesheet"/>
<div id="slides" class="carousel slide" data-ride="carousel">
<ul class="carousel-indicators">
<li data-target="#slides" data-slide-to="0" class ="active"></li>
<li data-target="#slides" data-slide-to="1"></li>
<li data-target="#slides" data-slide-to="2"></li>
</ul>
<div class="carousel-inner active">
<div class="carousel-item active">
1
</div>
<div class="carousel-item">
222
</div>
<div class="carousel-item">
3333
</div>
</div>
<a class="carousel-control-prev" href="#slides" role="button" data-slide="prev">
<span class="carousel-control-prev-icon" aria-hidden="true"></span>
<span class="sr-only">Previous</span>
</a>
<a class="carousel-control-next" href="#slides" role="button" data-slide="next">
<span class="carousel-control-next-icon" aria-hidden="true"></span> <span class="sr-only">Next</span>
</a>
</div>